stagefright: Resurrect PCM offload

 * Forward-port the open-source code from L which was moved to closed-source
   in M. This is being done out of necessity- the architecture chosen by
   Qualcomm is not optimal and doesn't work well with a singular
   codebase which attempts to service a large number of devices.
 * This patch brings in the code to support PCM offload (AudioFlinger
   bypass). This allows for playback of high resolution clips without
   decimation stages, and enables reduced power consumption for audio
   pipelines which take advantage of the Hexagon DSP (effects).

Change-Id: I0ef15fc3df538ab723f3c12ce0ed71d0e607c99e