Fix issue #34327404: AppOpsManager.stopWatchingMode(callback)...

...does not remove all references to callback

Keep the callbacks in a set, so each callback can only appear once.

Test: none currently, not sure how to do a CTS test for this.
(But verifying the system boots and runs.)

Change-Id: I01c8ea2a662e09ad0a0cdf713f0ea7f175182e82
1 file changed