TelephonyManager should use TelecomManager's getCallState().

Telecom's version includes non-cellular calls in its result. Many apps
compile against TelephonyManager's existing API so we should update it
to use telecom instead.

We kept the implementation as cellular-only for
getCallState[ForSubscription](...) version of the API (which is hidden
anyway).

Bug: 21732997
Change-Id: Ifd27e6b49d76fe96ccc969e5262b1a876670aeef