Don't use String8::empty

With the current definition, it's an alias of default constructor. This
has very different meaning from std::string empty, especially that
static method String8::empty can still be called from instance context.

One of such bugs are present in in ParametersUtil.

Bug: 295394788
Test: make checkbuild
Change-Id: I9116a0c42a6a9e94116eaaaa0fd60568db428838

audio/core/all-versions/default/ParametersUtil.cpp

@@ -51,7 +51,7 @@ Result ParametersUtil::getParam(const char* name, bool* value) {
     Result retval = getParam(name, &halValue);
     *value = false;
     if (retval == Result::OK) {
-        if (halValue.empty()) {
+        if (halValue.length() == 0) {
             return Result::NOT_SUPPORTED;
         }
         *value = !(halValue == AudioParameter::valueOff);