Add HEAD_TRACKER sensor type definition

This new sensor type supports tracking the orientation and rate of
rotation of a user's head, which can be useful for features such as
spatial audio.

Bug: 210156629
Test: compile only; to be validated in later CLs
Change-Id: I861be09d14e208fb992b23554b6c0733e4163f0c

sensors/aidl/Android.bp

@@ -1,5 +1,3 @@
-// This is the expected build file, but it may not be right in all cases
-
 aidl_interface {
     name: "android.hardware.sensors",
     vendor_available: true,

sensors/aidl/aidl_api/android.hardware.sensors/current/android/hardware/sensors/Event.aidl

@@ -51,6 +51,7 @@ parcelable Event {
     android.hardware.sensors.DynamicSensorInfo dynamic;
     android.hardware.sensors.AdditionalInfo additional;
     android.hardware.sensors.Event.EventPayload.Data data;
+    android.hardware.sensors.Event.EventPayload.HeadTracker headTracker;
     @FixedSize @VintfStability
     parcelable Vec4 {
       float x;
@@ -75,6 +76,16 @@ parcelable Event {
       float zBias;
     }
     @FixedSize @VintfStability
+    parcelable HeadTracker {
+      float rx;
+      float ry;
+      float rz;
+      float vx;
+      float vy;
+      float vz;
+      int discontinuityCount;
+    }
+    @FixedSize @VintfStability
     parcelable HeartRate {
       float bpm;
       android.hardware.sensors.SensorStatus status;

sensors/aidl/aidl_api/android.hardware.sensors/current/android/hardware/sensors/SensorType.aidl

@@ -70,5 +70,6 @@ enum SensorType {
   LOW_LATENCY_OFFBODY_DETECT = 34,
   ACCELEROMETER_UNCALIBRATED = 35,
   HINGE_ANGLE = 36,
+  HEAD_TRACKER = 37,
   DEVICE_PRIVATE_BASE = 65536,
 }

sensors/aidl/android/hardware/sensors/Event.aidl

@@ -127,6 +127,11 @@ parcelable Event {
          */
         Data data;
 
+        /**
+         * SensorType::HEAD_TRACKER
+         */
+        HeadTracker headTracker;
+
         @FixedSize
         @VintfStability
         parcelable Vec4 {
@@ -156,6 +161,46 @@ parcelable Event {
             float zBias;
         }
 
+        /**
+         * Payload of the HEAD_TRACKER sensor type. Note that the axis
+         * definition of this sensor type differs from the rest of Android. See
+         * SensorType::HEAD_TRACKER for more information.
+         */
+        @FixedSize
+        @VintfStability
+        parcelable HeadTracker {
+            /**
+             * An Euler vector (rotation vector, i.e. a vector whose direction
+             * indicates the axis of rotation and magnitude indicates the angle
+             * to rotate around that axis) representing the transform from
+             * the (arbitrary, possibly slowly drifting) reference frame to the
+             * head frame. Expressed in radians. Magnitude of the vector must be
+             * in the range [0, pi], while the value of individual axes are
+             * in the range [-pi, pi].
+             */
+            float rx;
+            float ry;
+            float rz;
+
+            /**
+             * An Euler vector (rotation vector) representing the angular
+             * velocity of the head (relative to itself), in radians per second.
+             * The direction of this vector indicates the axis of rotation, and
+             * the magnitude indicates the rate of rotation.
+             */
+            float vx;
+            float vy;
+            float vz;
+
+            /**
+             * This value increments (or wraps around to 0) each time the
+             * reference frame is suddenly and significantly changed, for
+             * example if an orientation filter algorithm used for determining
+             * the orientation has had its state reset.
+             */
+            int discontinuityCount;
+        }
+
         @FixedSize
         @VintfStability
         parcelable HeartRate {

sensors/aidl/android/hardware/sensors/SensorType.aidl

@@ -142,6 +142,10 @@ enum SensorType {
      * The rotation vector symbolizes the orientation of the device relative to
      * the East-North-Up coordinates frame.
      *
+     * Note that despite the name, SensorType::ROTATION_VECTOR uses
+     * quaternion representation, rather than the rotation vector representation
+     * (aka Euler vector) seen in SensorType::HEAD_TRACKER.
+     *
      * Implement the non-wake-up version of this sensor and implement the
      * wake-up version if the system possesses a wake up fifo.
      */
@@ -633,6 +637,35 @@ enum SensorType {
      */
     HINGE_ANGLE = 36,
 
+    /**
+     * HEAD_TRACKER
+     * reporting-mode: continuous
+     *
+     * A sensor of this type measures the orientation of a user's head relative
+     * to an arbitrary reference frame, and the rate of rotation.
+     *
+     * Events produced by this sensor follow a special head-centric coordinate
+     * frame, where:
+     *   - The X axis crosses through the user's ears, with the positive X
+     *     direction extending out of the user's right ear
+     *   - The Y axis crosses from the back of the user's head through their
+     *     nose, with the positive direction extending out of the nose, and the
+     *     X/Y plane being nominally parallel to the ground when the user is
+     *     upright and looking straight ahead
+     *   - The Z axis crosses from the neck through the top of the user's head,
+     *     with the positive direction extending out from the top of the head
+     *
+     * When this sensor type is exposed as a dynamic sensor through a
+     * communications channel that uses HID, such as Bluetooth or USB, as part
+     * of a device with audio output capability (e.g. headphones), then the
+     * DynamicSensorInfo::uuid field shall be set to contents of the HID
+     * Persistent Unique ID to permit association between the sensor and audio
+     * device. Accordingly, the HID Persistent Unique ID (Sensors Page 0x20,
+     * Usage ID 0x302) must be populated as a UUID in binary representation,
+     * following RFC 4122 byte order.
+     */
+    HEAD_TRACKER = 37,
+
     /**
      * Base for device manufacturers private sensor types.
      * These sensor types can't be exposed in the SDK.