MediaCodec: fix javadoc on onMetricsFlushed

Clarify that the callback is only used for metrics that are
flushed during the executing state, and describe how the
metrics can be retrieved during codec reconfiguration.

Change-Id: I9079920ae96556388f97b2d9b61fd9ae967c6c0f
Flags: DOCS_ONLY
Test: compile
Bug: 378167016