Allow releaseBuffer after flush

After AudioTrack start checks for pending flush,
allow releaseBuffer on any previously obtained buffer.
For example, this can happen if the resampler has obtained
a buffer but not released the whole buffer yet.
Note that the resampler will be reading obsolete data.

Bug: 11285590
Change-Id: I0614fbb62e43604aac3089cce4b7797c87a306b5