Add a SubscriptionModel

We currently only need 2 bits from `SubscriptionInfo`: subscriptionId
and isOpportunistic. This CL moves those into a `SubscriptionModel`
class and updates all of the usages.

Test: existing tests
Bug: 249790009
Change-Id: I87686213e68b199b7d4c998f02d6687dcb276a27
13 files changed