Implement QUEUE_FLUSH and QUEUE_DESTROY addition modes.

Note that this change is slightly more flexible, it defines
a priority for each playback request. We needn't really throw
away synthesis requests from other apps. We could just play
them after higher priority ones (for e.g from talkback).

Change-Id: I6c7dd9abe4871e87da08d9aaa4c55225a69078e5