Refactor startOutgoingCall in CallsManager

Modify startOutgoingCall to use futures to manage the asynchronous
elements of starting an outgoing call (e.g. phone account selection,
prompting for user input, etc). This is done in preparation for adding
the phone account suggestion feature.

Bug: 111455117
Test: unit, manual
Change-Id: Ia0074738b7c4b146b3c9dbb6e425fe43bb0ac122