Multi-endpoint changes.

Passing the EXTRA_IS_CALL_PULL extra directly into the ImsCallProfile.
mCallExtras; this is where the modem looks for it, not in the "intent
extras".

Adding unit test for ImsExternalCallTracker (still more tests to be added
in the future); this required adding a new interface to abstract away
some stuff that is needed for the tests.

Bug: 27458894
Change-Id: I24b20d13d5dfeb422295d80c0eec72800836ae74