Avoid crash when resetting defaults.

Tearing down means unregistering receivers, but if the receiver isn't
registered, then the system will crash. Now, we make sure the receivers
were registered before trying to unregister them.

Bug: 158300259
Test: manual via developer options
Change-Id: I3a022002e2f1286718e08fe1dddf83deda6290de
3 files changed