Changes to SubscriptionController to allow multiple sims per slot-index.

Allowing multiple subscriptions per slot-index may need changes
elsewhere in the code. But this CL only deals with a local data struct
in SubscriptionController.java.
This CL's immediate beneficiary is work associated with b/110267643

Bug: 112321331
Test: tested w/ unittests
Change-Id: Icae42dce43c4557a656f8ab6635bed5ebcc44730