APIs to watch active op changes

System singed components can watch for starting/finishing of
long running app ops. Also protected the APIs to watch op mode
changes with a singature permission for the cross-uid use case.

Test: atest com.android.server.appops.AppOpsActiveWatcherTest

bug:64085448

Change-Id: Id7fe79ce1de4c5690b4f52786424ec5a5d9eb0fa