SubscriptionController: do not return info for invalid slot id

This causes Keyguard to crash when you switch a dual SIM phone into
single SIM mode for testing because the old data with the now-invalid
slotId is stored in the database.

Change-Id: I9d6102f94c11b808c272111230322507408b2d87
(cherry picked from commit 7f3b180ecf6b0094d9dd80ced579176eb74f6686)