Register OnSubscriptionsChangedListener and cache results.

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
(cherry picked from https://android-review.googlesource.com/q/commit:c48d856976c9fcb8150f4de524d69cc5d68ce46d)
Merged-In: I889d4da725ccda713367309c257622a0bf9939f3
Change-Id: I889d4da725ccda713367309c257622a0bf9939f3