[Sb refactor] Implement 2s grace period for data switching

The old mobile pipeline supported this particular use case. Given that
there are 2 mobile subscriptions, A and B, then when data switches from
A to B, we will consider B to be validated (see NetworkCapabilities) for
up to 2 seconds under the conditions:

1. A was validated before the switch
2. A and B are both in the same group (i.e.  they have the same group UUID)

This CL implements the same exact criteria by monitoring for data
switching in the same group and maintaining a validated state for 2
seconds for the default subscription.

Test: MobileIconsInteractorTest
Bug: 238425913
Change-Id: I8e798197beda562634c137377145dea510cbda4c