Check that modes can be deleted before adding delete menu option.

Migrates to using MenuProvider instead of onCreateOptionsMenu, since the previous approach is apparently now deprecated.

Also wraps usages of requireActivity() so that we're sure the activity isn't null instead of potentially throwing an exception.

Flag: android.app.modes_ui
Bug: 346575126
Test: manually confirmed that manual DND is no longer deletable
Change-Id: I872f6054061c019db9a72028cc90cbb123a1cdce