Enhance PhoneAccount selection to take video capability into account.

Enhance PhoneAccount selection logic to take into account which phone
accounts can place video calls when placing a video call.
As part of this work, moved that selection logic in startOutgoingCall into
a new method and set up a CallsManager unit test class which tests the
phone account selection logic.

Test: Added unit tests, performed manual tests using test app.
Bug: 63939933
Change-Id: If79131b311ca19ae6f6cc2878892398633315c63