Unregister BroadcastReceiver and OnSubscriptionsChangedListener.

1. Unregister BroadcastReceiver of default subscription id.
2. Use a CompletableFuture to store and unregister the
   OnSubscriptionsChangedListener. Note this has to be done since the
   listener cannot be constructed in the handler thread.

Bug: 288059409
Test: atest FrameworksNetTests
Change-Id: Ia5fc3e53305a99c32ad2f6d5b1b6a367dc20c1d7
2 files changed