Pipe TelephonyManager.getCallState through TelecommManager

* Add TelecommManager.getCallState (hidden API)
* Make TelephonyManager.getCallState call through to
TelecommManager, to be consistent with
TelephonyManager.ACTION_PHONE_STATE_CHANGED broadcasts for
overall call state. Telephony continues to manage call states for
individual subscriptions.

Bug: 17378767
Change-Id: Ia5e8b21df801ed3af4f6e14c110a72c92f077f88