libstagefright: Restore AOSP's handling of 24/32-bit wav files

Presumably Steve changed this to make it better, but since it is
broken lets restore the downsampling code from AOSP and at least the
audio tracks won't be corrupted until someone really fixes what he
was trying to backport here.

This fixes, for example, Google Allo's notification sounds being corrupt
(static).

Change-Id: I58a0d4e4ea8650014919c95b6f9895981db93208