AudioTrack: Set DIRECT track flag for direct pcm output

While direct output should ideally be abstracted from the AudioTrack,
the use of an offload path can leave data stuck in the shared buffer
(especially if the clip is short such that the buffer is never
entirely filled). To ensure that all data is played out on track
stop(), AudioTrack must be aware of the direct output or offload
session. Set the DIRECT track flag for track offload use cases to
avoid losing data in the shared buffer.

CRs-Fixed: 944878
Change-Id: I13f840b5d67be56e03ec65d3bc7d44f1090b48c5