Add subscription specific API for IMS registration status

Add a new API "isImsRegisteredForSubscriber(int subId)" so that IMS
registration status for specific subscription is acquired.

Bug: 28806101
Test: Manually Enable/Disable IMS and view Settings
Merged-In: I6c452c512f03cf41704b91331e44141ed3050cf9
Change-Id: I6c452c512f03cf41704b91331e44141ed3050cf9
1 file changed