pcm: Make more of the low-level PCM API private
* pcm_play_data
* pcm_play_stop
* pcm_play_stop_int
* pcm_is_playing
* pcm_set_frequency
* pcm_get_frequency
* pcm_apply_settings
Now, the only user of these functions are the mixer and recording layers
that provide a higher-level API to plugins and the main [playback]
application.
Outside of the PCM core, pcm_apply_settings() was only used immediately
following a call to mixer_set_frequency(), so the latter function
now always calls the former.
Change-Id: I61c3144dc156b9de9b7963160b525c6d10c6ad4b