stagefright: Improve FLAC and WAV support for 24-bit

 * Forward-port changes from CM11
 * Remove truncation support as AudioFlinger handles
   rebuffering in case it can't go to the hardware.

Change-Id: Iad30d04ee051050e444c3d665fa8bb7a1cfef348
audioflinger: Enable floating point samples in AudioFlinger

 * Enable "new" mixer mode to enable mixing and reformatting
   of high precision (> 16-bit) streams.
 * Add missing 8_24 format to list of allowed formats.

Change-Id: I7d7b8ca7e34b25afda3af7e796d24cb94779715a