stagefright: Support for 24-bit audio in StageFright

 * Plumb bit depth thru ACodec and OMX
 * Add support for 24-bit PCM offload in NuPlayer on QC devices
 * Use new AudioFlinger features for mixing multi formats without offload
 * Clean up a bunch of code

Change-Id: I018d3a995b63450a38c6c43eaa37c86be30fd893