stagefright: 24-bit audio support via offloading

 * PCM offload feature can handle 24 bit streams padded to 32 bits,
   bypassing resampling and truncation.
 * Add support for 32-bit padding in FLAC and WAV decoders
 * Refactor the FLACExtractor a little
 * Set flags where appropriate
 * Tested with 96KHz/192KHz 24-bit FLAC files on msm8974pro
 * Audiophiles, rejoice!

Change-Id: I87bfabd991da34fbb6115e69ee5a935d941c29da