Initialize usb_count in the setup funtion.

Spurious wakeups can cause the test thread to return even if the
callback was not actually called as the usb_count isnt initialized.

Bug: 65469351
Test: mma and pushed locally to the device & tested.
--skip-preconditions --module VtsHalUsbV1_0Target
Change-Id: Ib0e838cf4a44807142eab6aa5e9df0cc462bb973

usb/1.0/vts/functional/VtsHalUsbV1_0TargetTest.cpp

@@ -160,7 +160,7 @@ class UsbHidlTest : public ::testing::VtsHalHidlTargetTestBase {
   // and the callback thread.
   std::mutex usb_mtx;
   std::condition_variable usb_cv;
-  int usb_count;
+  int usb_count = 0;
 };
 
 /*