Add @hide API to get the phone account handle for a subId.

This is used in SubscriptionManager to figure out which phone account
is being updated when the default voice sub changes.  It seemed like
a good spot to put it since we should really have an api for this in
the future.

Test: Manual, manual, manual
Bug: 128916238
Change-Id: If5137c0e7f01a14810e0ee94e5d857d69f368a87