Add a helper class for filtering available subscriptions

In several places implementing the new changes for better UX with
multiple mobile subscriptions, we need a bit of code that filters the
results coming from the SubscriptionManager. This CL adds that common
code in a util class that will be used in later CLs.

Bug: 116349402
Test: RunSettingsRoboTests
Change-Id: Ie9376629604ac2c4a2f15123edc380c5999f2f83