Miscellaneous updates to <system/audio.h>

Add single-precision floating-point PCM format:
    - AUDIO_FORMAT_PCM_SUB_FLOAT
    - AUDIO_FORMAT_PCM_FLOAT

Add symbols to replace hard-coded constants in code:
    - AUDIO_IO_HANDLE_NONE
    - AUDIO_IN_ACOUSTICS_NONE
    - AUDIO_INTERLEAVE_*
    - AUDIO_STREAM_MIN

Update comments.

Fix whitespace in switch statement.

Change-Id: I61862c188ebac06657b5d9887164edc24b8ab8f5