AudioTrack: Allow flush in flush state to clear subsequent written data

Fixes Photos a/v sync issue introduced by
commit 1d3556d6

Test: Photos, pause, then seek several times, then play.
Test: CTS AudioTrackTest, Bluetooth, Offload, Gallery, Play Music.
Bug: 79327282
Change-Id: I9de8edd88b526b5a9dbc9cd720bf725474949dfc