SDK / exoplayer

When a passthrough AudioTrack is replaced (due to seeking) the new one behaves
as if it is still emptying data from the old one, with its playback position
advancing until it runs out of data.

Data written while the 'old' AudioTrack was emptying would be discarded, so
avoid writing to the new AudioTrack while the old one is still emptying.

Also avoid using AudioTrack.getTimestamp with passthrough tracks, as this causes
the playback position to jump to a position that breaks audio/video
synchronization.