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: I66716314a2e5679e5a4fb7a9e429ae547bd0951d
Change-Id: I66716314a2e5679e5a4fb7a9e429ae547bd0951d