plugins: Convert all plugins to using mixer API for playback

pcm_play_data -> mixer_channel_play_data
pcm_set_frequeny -> mixer_set_frequency
pcm_play_stop -> audio_stop at startup / mixer_channel_stop at shutdown
pcm_is_playing -> mixer_channel_status

All of these have been removed from the plugin API.  Updated API docs to
clarify role of audio_stop() vs mixer_channel_stop()

Todo:  get rid of pcm_play_[un]lock().

Change-Id: I8800c131b51f064ee923a4c6e42aa405d33851fc

apps/plugin.c

@@ -645,10 +645,6 @@ static const struct plugin_api rockbox_api = {
     &audio_master_sampr_list[0],
     &hw_freq_sampr[0],
     pcm_apply_settings,
-    pcm_play_data,
-    pcm_play_stop,
-    pcm_set_frequency,
-    pcm_is_playing,
     pcm_play_lock,
     pcm_play_unlock,
     beep_play,