Rename MobileSubscriptionModel to MobileConnectionModel

Also rename `subscriptionModelFlow` to `connectionInfo`, which is more
clear given that `MobileConnectionRepository` is the provider of that
information.

This paves the way to remove `SubscriptionInfo` from our internal
representation in lieu of a `SubscriptionModel` based off of it

Test: tests in tests/src/com/android/systemui/statusbar/pipeline/mobile/*
Bug: 261029387
Change-Id: If4bd3e31d2f8a7400e31e4acd8776750eee22ad6