Restricts notified app ops based on flags

AppOps that are received by SystemUI and notified to listeners of
AppOpsControllerImpl are filtered based on the permission flag
PackageManager.FLAG_PERMISSION_USER_SENSITIVE_WHEN_GRANTED.

As calls to obtain this flag require an IPC, three things are done to
mitigate impact:
* PermissionFlagsCache keeps track of requested flags and will update
those (in a background thread), when a change is notified for a given
uid.
* Calls to getActiveAppOps/getActiveAppOpsForUser should be made from a
background thread.
* notifySubscribers is always called in the background thread.

Bug: 160966908
Test: atest PermissionFlagsCacheTest AppOpsControllerTest
Change-Id: I871094c32ce5ec940d779626333caa0ca500a4e3