audioflinger: fix for playback paused during track transition

The test case here involves two applications both of
which play the same music clip. When the 2nd application
ends playback, the 1st application must resume playback
where it left off. But it is possible that the new
AudioTrack.start() command from the 1st application is processed
after the 2nd application has paused and terminated.

This will prevent a flush to clear any state before playback
of the 1st application can resume. Direct thread thus
stays in pause state indefintely.

Issue unconditional flush for direct thread when a new
track is added to a direct output.

Test: partner CL submit, check compilation
Bug: 123082418
Change-Id: I33d36db35358a1b3b89dd5b2b8cb4db1f9c6d4a0