Update SubscriptionInfo o use strings for mccmnc

Add methods to get mcc/mnc as strings so that the leading-zero
ambiguity is resolved.

Test: manual (db update), unit tests
Bug: 35064313
Change-Id: I45c0bfefb7ffe98e3eab8e68d0e1170881ae9f4c
Merged-In: I45c0bfefb7ffe98e3eab8e68d0e1170881ae9f4c