Push initial pair to frame map when track starts from PAUSED state

This makes the findX() return constant trackframe when the sinkframe
is between the pause and resumption. The application will not see
unexpected incrementing trackframe from getTimestamp() before the time
elapses by "output latency" after resumption.

This fixes AV desync observed in the first 10 seconds after resumption
in ExoPlayer demo app.

See go/exoplayer-resume-desync for more details.
Bug: 377842501
Test: observe no desync when resuming from ExoPlayer demo app
Flag: EXEMPT bugfix

Change-Id: Ic5cda8f8e06f1cc9a6f6494989f6c62b61127162