stagefright: Handle offload fallback for 24-bit tracks

 * We need to carefully handle fallback in the 24-bit lossless
   sources for outputs such as A2DP, which cannot use PCM
   offload.
 * Clean up the code while in here as well.

Change-Id: I398d964a3c5b18f58fcc79376bc29222e796badc
Merge remote branch 'cm/cm-11.0' into cm-11.0