CoverAsyncHelper: Fix warnings.
MPDroid: Don't pass settings to get pm.getDefaultSharedPreferences().

This can easily be acquired globally where needed with the singleton.
MPDroid: Remove arguments from AlbumCoverDownloadListener constructors.
MPDroid: More singleton cleanups.
MPDroid: Allow controls at all times by intent.

Move control intents actions to RemoteControlReceiver. This allows media
buttons to work at any time, simplifies NotificationService, and other
advantages.