Added empty subscription manager service

Non-functional skeleton. The methods are added because we can't
remove methods in ISub.aidl at this point. Those are still used
by the to-be-removed SubscriptionController.

Bug: 239607619
Test: Manual
Merged-In: Iffc26de09a8da75b052f06399cf033b2df1d3ec8
Change-Id: Iffc26de09a8da75b052f06399cf033b2df1d3ec8