MediaCodec: add onMetricsFlushed callback API

This is used to notify when metrics are flushed due to the start of a
new subsession, allowing the client to capture the metrics for the
previous subsession.

Bug: 363382811
Test: build and boot
Flag: android.media.codec.subsession_metrics
API-Coverage-Bug: 378167016
Change-Id: I324711e4da565d15a1bb9baed74dffa695a41bc0