Settings: Don't crash if we pause too quickly

Notifier is unregistered in every case in pause.
However, if you close the Settings app too quick, the notifier
might not be registered at all, causing an IllegalArgumentException
during the unregistering.

Change-Id: I54141cbfc91d08c28ef524f4155b673ae8dee90e
Merge "Settings: Don't crash if we pause too quickly" into android-4.4
Merge branch 'android-4.4' of https://github.com/omnirom/android_packages_apps_Settings into android-4.4