Register OnSubscriptionsChangedListener and cache results. am: 16800bf437

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/23720722

Change-Id: I6ae84b3478e1d7d85c9d36cfa3a4e78e2cb6b082
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>