Add device and carrier flags for video calling fallback

These flags let the system dialer/contacts/SMS app determine if
integration with video calling apps are allowed if the carrier video
calling is not available.

EXTRA_SUPPORTS_VIDEO_CALLING_FALLBACK in PhoneAccount.getExtras()
should be checked first. This flag is disabled by default and OEMs
will need to enable it.

If the device supports the fallback, then
CarrierConfigManager.KEY_ALLOW_VIDEO_CALLING_FALLBACK_BOOL should be
checked so individual carriers can ban the feature. This flag is
allowed by default.

Change-Id: Id46f684254402acb9142f87684ddff2d830f1701
Fixes: 62264468
Test: manual