stagefright: Improve 24-bit support

 * Cleanup and fixes to allow FFMPEG codecs to output raw 24-bit PCM

Change-Id: Ideeb7cd7c6340f8054f99272796032939b561f81
audioflinger: exchange AUDIO_SOURCE_HOTWORD with AUDIO_SOURCE_VOICE_RECOGNITION for pre-kitkat blobs.

Change-Id: I6241cf6667c97dfd7fb1c238b1fbadea5cdb863e