Change SubscriptionController to make subscription unique by ICCID

We found that as a consequence of b/170682350, the siminfo table
maintained by SubscriptionController has actually somehow gotten multiple
rows with the same ICCID inserted. Thus, we exam the ICCID before
inserting a subscription info record.

Test: atest SubscriptionControllerTest#testInsertEmptySubInfoRecord_returnsNull_ifRecordExists
Bug: 170994576
Change-Id: I0e486e08735707a23c2ece66b2c6b6db356b6a55