Fix Bug in ImsManager#addRegistrationListener

Listeners were not being unregistered from
the ImsService. The listeners were restructured
to instead be held in ImsManager, instead of the
ImsService.

Bug: 38304804
Test: Run Rogers VoWiFi activation
Merged-In: I2a0d4b2ed7d232834be7c74c318a60fa11e64389
Change-Id: I2a0d4b2ed7d232834be7c74c318a60fa11e64389