Add ImsManager public API and refactor to use API.

Modified ImsManager to "simulate" the new ImsMmtelManager
functionality until the framework can be refactored to
remove ImsManager entirely.

Test: atest FrameworksTelephonyTests
Merged-In: Id754f07e132e641f930e638ea06770ac1e8cae57
Change-Id: Id754f07e132e641f930e638ea06770ac1e8cae57