This commit rearranges result codes, changes their constants and
removes unused codes. If you're a binding maintainer you will need to
update your result code constants.

Future commits may change these further.

Removed APIs:
 * mal_device_set_recv_callback()
 * mal_device_set_send_callback()
 * mal_src_set_input_sample_rate()
 * mal_src_set_output_sample_rate()

Renamed APIs:
 * mal_sine_wave_read()    -> mal_sine_wave_read_f32()
 * mal_sine_wave_read_ex() -> mal_sine_wave_read_f32_ex()

Renamed APIs:
 * mal_decoder_read()          -> mal_decoder_read_pcm_frames()
 * mal_decoder_seek_to_frame() -> mal_decoder_seek_to_pcm_frame()

This backend is the new priority backend for Android.

This makes it more convenient to test because it avoids the need for
the use of emrun.