Throw IllegalStateException when IMS is not ready for callback registration

Simulate existing behavior for when IMS is not ready by throwing an
IllegalStateException until IMS callbacks can be registered. This will
ensure there is no way that a callback can be registered and then removed
invisibly once the IMS stack catches up to the state of telephony.

Bug: 219529364
Test: atest FrameworksTelephonyTests; manual
Change-Id: Ic0e1ea2c9ab81dd9c7bb99825be317f8703a5f67