Public API to watch for "active" operations.

We already have startOp() and finishOp() to provide a mechanism to
mark an operation as being "active".  This change publishes a set of
APIs to check the status of these active operations, matching the
naming of similar existing APIs on the same class.

Bug: 139128842
Test: atest com.android.server.appop.AppOpsActiveWatcherTest
Test: atest android.media.cts.AudioRecordAppOpTest
Exempt-From-Owner-Approval: trivial API refactoring
Change-Id: I9fb381d748ff23ff24dd363ed5b117bd661793ab