Integrate IMS Registration API

Integrates the new IMS Registration API into the framework
and add two APIs for feature parity.

1) Moves ImsRegistrationImplBase to the android.telephony.ims
namespace
2) Expose ImsRegistration AIDL in TelephonyManager as hidden
API.
3) Adds onVoicemailCountUpdate and
onSubscriberAssociatedUriChanged APIs for feature parity.

Bug: 63987047
Test: Telephony unit tests, manual IMS tests
Change-Id: I51a8ae5cf58b81708c21adc1b26a9eb59f6280fd