audioflinger: fix track terminated before playing

When a track is terminated before any audio was written,
we should not send a drain request to the HAL. The drain
will be ignored and we will wait the async callback for ever.

Also fix a potential deadlock by not deleting a track with the
thread mutex held: mPreviousTrack changed from a sp to a raw pointer.

Bug: 11247103.
Change-Id: I0a4b47b099204c7c326d45d700234ab8f72d8eae