Add SystemApi for registering Usage Session Observers in UsageStats

Test: atest cts/hostsidetests/devicepolicy/src/com/android/cts/devicepolicy/ProfileOwnerTest.java#testAppUsageObserver
Test: atest android.app.usage.cts.UsageStatsTest#testObserveUsagePermission
Bug: 111465038
Change-Id: I70a7a5729117f8e9651a2e4ea732156537d2f3c1