[rbp/omxplayer] Avoid resampling in audio_mixer

audio_mixer output defaults to 44.1kHz. Set it to output of audio_decode to avoid a resampling step
This restores the behaviour prior to #2801
Merge pull request #2917 from popcornmix/avoid_resampling_mixer

[rbp/omxplayer] Avoid resampling in audio_mixer