base

Remove CallServiceProvider and CallServiceDescriptor

This CL removes CallServiceProvider. Instead of using a
provider we'll look up ConnectionServices using the package
manager instead.

This CL also removes CallServiceDescriptor. For incoming calls,
connection services will now pass a PhoneAccount instead
of a call descriptor. For outgoing calls we already use
PhoneAccounts for everything.

Change-Id: I2e40c5c64c0d242dc41b680943d7e9209142db5b