Refactor the holding logic in ImsPhoneCallTracker

Modify ImsPhoneCallTracker to replace mSwitchingFgAndBgCalls with an
enum that describes the exact operation that is being performed. In
association with this, remove the switchWaitingOrHoldingAndActive method
because it's a holdover from the GSM/CDMA behavior.

Also perform some cleanup of CallsManager.java -- removed some old code
and deprecated some APIs.

Bug: 75608720
Bug: 115943523
Test: unit tests, manual tests

Change-Id: Id0b9664bacb2fbedaeffb50694e4c12e81b97c39