SDK / exoplayer

Currently the audio renderer can become ready before the AudioTrack
actually has enough data to play something, which means that the
player may transition to the ready state before audio starts
playing. This makes the player's current state transition not very
useful for detecting when audio actually starts playing.

This change adds a new event to notify apps when the audio position
is increasing after a pause or seek/flush/reset event, and includes
an estimate of the system time at which audio playout started.

Issue: #7577
PiperOrigin-RevId: 327810040