stagefright-plugins: Use the max aggregated buffer size

 * NuPlayer has a new feature which aggregates small buffers before
   delivering them to the codec. We need to match this size in all
   cases or we will have a bad time.

Change-Id: I353cedbe6f864e89214f185bbffa7314d7caf77c
stagefright-plugins: Enable 24-bit output

 * Always enable it. Audioflinger has conversions for when the hardware
   can't support it.
 * Also fix a bunch of state management issues

Change-Id: I2106b34251bd0c9db0655dd97d3e92c7b635a982