Further work on libmediametrics stable API

implement a missing method (for inflight metrics), enumerate exported
symbols. Reconcile common source files
core/jni/android_media_MediaMetricsJNI and media/jni/android_media_MediaMetricsJNI
use new "libmediametrics_stable" library for mediaplayer2

Bug: 119675363
Test: build, CTS media/codec tests
Change-Id: Ide72dfe891231c3764329686955e1b4682094b1e