Properly protect mFrameMetricsReporter

This field actually requires a special lock, mFrameMetricsReporterMutex.
But there isn't a GUARDED_BY annotation for it. And even if there was,
the compiler feature of -Wthread-safety was not active in this code, so
this error would not have been caught.

To fix this, enable the compiler annotation and add GUARDED_BY
annotation to mFrameMetricsReporter.
And finally, use this lock to properly protect this field.

Bug: 192330836
Test: atest hwui_unit_tests
Change-Id: I76950bfa01bbd7ccdc54c4e8c114430b5aeddf1a