Allow the supervision app to access certain UsageStats APIs.

Allow the active supervision component to access app usage limit APIs.
This includes #registerAppUsageLimitObserver
and #unregisterAppUsageLimitObserver.

Bug: 138681869
Test: atest com.android.server.devicepolicy.DevicePolicyManagerTest#testIsActiveSupervisor
Test: atest AppTimeLimitControllerTests
Test: atest android.app.usage.cts.UsageStatsTest
Change-Id: I3daafc14ccdbf27558c1325b965f2bc6d2dab2f6