Add CAPABILITY_EMERGENCY_PREFERRED in PhoneAccount

Allow Telephony to mark a SIM PhoneAccount as
emergency preferred, meaning that Telecom will
override a user's PhoneAccount preference for
emergency calls if the PhoneAccount has the
CAPABILITY_EMERGENCY_PREFERRED capability.

Bug: 131203278
Test: Manual testing, Telecom/Telephony unit testing
Merged-In: If70d917cfb3d825f375d282012b8788d69a2f144
Change-Id: I88b8bbfa444f5445b2f0d6a1542c6406a19b240f