Add API to listen to opportunistic subscriptions change.

Add a new set of APIs so that caller can listen to
opportunistic subscriptions changes.

Bug: 92796390
Test: build
Change-Id: Iaa741141bf1bd04c4b1618d5bfeac733a0690ed8
Merged-In: Iaa741141bf1bd04c4b1618d5bfeac733a0690ed8
4 files changed