Introduce Telephony2gUpdater to Apply 2g User Restrictions

Listens for the following actions and ensures that 2g is enabled
or disabled based on the state of the `DISALLOW_2G` user
restriction. 2g is enabled or disabled for all active subscriptions
on the device.

* A broadcast for action `android.os.action.USER_RESTRICTIONS_CHANGED`
* A callback from `SubscriptionManager` informing us of a subscription
state change

Test: atest Telephony2gUpdaterTest
Bug: 230669497
Change-Id: I819d412bc14cb98a5874d2a42b50d4e4483b4b02
4 files changed