Use AudioManager API instead of manually checking ringing volume.

This CL ensures that the Ringer class relies on the AudioManager#shouldNotificationSoundPlay API instead of manually checking ringing volume when determining if a ringtone should be played audibly for an MT call. This CL also adds the feature flag for all of the go/ring-my-car Telecom feature work.

Bug: 348707841
Flag: com.android.server.telecom.flags.ensure_in_car_ringing
Test: atest RingerTest

Change-Id: I7df60d54e4e6e8ede69e807a6091647104ea9863