SubscriptionController: getPhoneId() should never return negative values

Most places dealing with getPhoneId() check for negative returns and use
zero if it happens. Instead of doing that, when dealing with negative subIds
generate positive slot identifiers by replicating the logic from changes
I91d99c98cff9b6fd484d4972b5461098bba23ba2 and Id013b2673645b63db8c96660e5a9446347a3e2e5

Change-Id: I4577764298cd53199d036d40b58b6c9490c2bacb