Prepare for additional formats beyond 8-bit and 16-bit PCM

Factor out code that converts from Java ENCODING_PCM_* to native
AUDIO_FORMAT_* into a new function audioFormatToNative().

Also use audio_bytes_per_sample() from <system/audio.h>
instead of hard-coding the size of various formats.

Use size_t for memory sizes.

Change-Id: Ic3525f049e939bdf125d9f87ed39abd2690dcf9f