Move receiver registration to onPause/onResume for MobileNetworkSettings.

Moving registration and unregistration of receivers on the
MobileNetworkSettings page to the onPause/onResume methods.
Previously this was done in onCreate and onDestroy.
Also, adding null checks in the receivers to ensure that they won't try
to update the UI when the activity is null.

Test: Manual
Bug: 109899326
Change-Id: Ic5a23c33d8162152e99e033eab9046cd70051482
1 file changed