Implement CAPABILITY_EMERGENCY_PREFERRED capability

When a PhoneAccount has CAPABILITY_EMERGENCY_PREFERRED
defined and an emergency call is placed, override
the user's choice of PhoneAccount with this account.

Bug: 131203278
Test: atest TelecomUnitTests
Merged-In: I0c0ae21c38f526eff801967bf45137701446115d
Change-Id: I0381ffede54d0a662feb2dade0859032f200ea8a
5 files changed