Register a listener to get notified of SubscriptionInfo changes and store all carrierIds of active subscriptions in a cache. The executor for the listener callback runs on a different thread to the connectivity thread but posts the SubscriptionInfo list to the connectivity thread for caching. Bug: 273451360 Test: atest FrameworksNetTests Change-Id: I889d4da725ccda713367309c257622a0bf9939f3