audio: add support to enable Direct PCM output

This change squashes all changes done to enable
direct PCM output.

audio: use offload standby delay for direct pcm
Change-Id: Ifd0e39c5b3fe4a4af9f444dbbad21ef1e1a5edfa

AudioTrack: Set DIRECT track flag for direct pcm output
Change-Id: I13f840b5d67be56e03ec65d3bc7d44f1090b48c5

Direct_PCM: add support for effects
Change-Id: I2fbac63c623bf51a03e5e91828369739d33329f3

libmedia : audio: handle set playback rate when direct pcm used
Change-Id: I6df7c10b7115c77a19564e2f9fabc511d76a2410

audioflinger: Always process volume commands from a latest track
Change-Id: I9afbdd221db3a7311eb0ec8bb75852cf5409084e

Change-Id: I2ad7eacf11642a4ca9f892b61124293d0dc503a9