Helper function to return the unique name for subscription info

Many classes within Settings have access to the subscription info, rather
than only having access to the subscription id. For these classes, it is
more convenient to have a helper method where the whole subscription
info can be passed in, rather than having to call the
getSubscriptionId() helper method in so many locations within the
telephony settings codebase.

Bug: 150370656
Bug: 148303118
Test: atest -c SubscriptionUtilTest
Change-Id: I181960933ab8610d5572b7a7d6671fab8c1dbd22