(Android) Add setEnabledMedium to (un)mute bidirectional audio or video

Usage:

  cordova.plugins.phonertc.setEnabledMedium('audio', false);
  cordova.plugins.phonertc.setEnabledMedium('audio', true);

  cordova.plugins.phonertc.setEnabledMedium('video', false);
  cordova.plugins.phonertc.setEnabledMedium('video', true);

See #32