Adds a tappable 'favorite' custom media control to the Android Auto
notification, replacing the less useful stop control (skip/pause/play/
skip already cover playback stop).
audio_service has no built-in favorite action, so this follows the
same custom-control pattern already used for shuffle: a custom action
name is wired up in customAction(), and _transformEvent() swaps the
control's icon (ic_favorite / ic_favorite_on) based on whether the
active track is currently saved, via the metadata plugin's saved-
tracks provider.
