Replace CallServiceSelectors with Subscriptions (1/3)

Remove CallServiceSelectors and replace them with comprehensive
support for Subscriptions as the means of selecting ways of making
phone calls. After this change, a ConnectionService is not a
semantically meaningful "way of making a call" -- it's more like the
mechanism whereby the Android system communicates with a 3rd party
process to ask for phone services. We anticipate each process having
only one ConnectionService.

Change-Id: I11e6e246ae999683b3800496e98c93c3351aca7b