Add flag DIRECT to track flags for track running on DirectOutputThread

If the playback thread is DirectOutputThread, add
AUDIO_OUTPUT_FLAG_DIRECT to the track's flags. This allows HAL to
receive `Flush`, as well as many other control signals, when client
sends the commands.

Bug: 111196161
Test: On device
Change-Id: I48fae1e05cfb7aabf53a050c72ec78d18e478b0a