Fix ImsManager double callback registration

MMTelFeature has two ways of registering registration callbacks,
through startSession and addRegistrationListener. startSession
requires a registration listener, which can cause double
registration if addRegistrationListener happens before
startSession in ImsManager.

This fix temporarily solves that problem by registering a Stub
implementation of the registration callback in startSession.
This will be fixed later by modifying the MMTelFeature
APIs to remove the registration listener from the arg list.

Bug: 62207372
Test: Telephony unit tests and manual IMS registration
Merged-In: Ieeff249e05777bdb9135e9ddd6e08a15416d5a1f
Change-Id: I00fdbae1baaf77cca5156a442ea109f998a8d5a5