stagefright: More high-resolution audio support

 * Clean up the "error driven" fallback code we had before and
   reconfigure components if the output bit width needs to change
   (a2dp/usb fallback).
 * Decode various formats (AC3/DTS/EAC3) which output floating point
   samples using FFMPEG, which will output 24-bit PCM for us.

Change-Id: I94f5fa59f5f737be8969df4c5ad4295c45cb8657