Add stub implementation of adding reg and cap callbacks.

To unblock SMS/IMS reg integration.

Test: manually testing failure and retry logic only now
as testing success case is not yet feisble.

BUG=69846044
Merged-In: I7b6066306a09db9dfd0f332661ea078407c5a960
Change-Id: I7b6066306a09db9dfd0f332661ea078407c5a960