Settings: drop Lollipop-specific styling

DeviceDefault (from values-v14) will be Material on Lollipop, which is the goal.
No reason to explicitly declare this as AppCompat or Material in values-v21.
Service: set new Notification properties for Lollipop
Service: cancel lingering AsyncTask in onDestroy() if present

This should not be necessary, but it is an attempt to fix an obscure bug that I
cannot seem to reliably reproduce where the app crashes at runtime due to a
thrown ConcurrentModificationException. I think this only occurs in test/debug
when installing different builds, but there's no harm in checking for a running
task anyway.