Introduce CaptureStateNotifier

This is a helper class for notifying a client of capture
state changes.

Bug: 146157104
Test: Compiles (further testing as part of the topic).
Change-Id: Ie27bc404805d319a6969290a6369d59fb68c1f38
Merged-In: Ie27bc404805d319a6969290a6369d59fb68c1f38
5 files changed