Essential packages' notifications can no longer be silenced.

Particularly useful for Dialer, whose incoming call
notifications are basically useless if you accidentally mute
them.

Test: manual. With config_nonBlockableNotificationPackages
      set to include com.android.dialer, receive or miss a
      call and longpress the notification. No radio buttons
      should be available.
Bug: 31360343
Change-Id: I6b8fa374fb8e811ec6922acbf222f23f18775a61