Added APIs for App Usage Limits.

Added a new AppUsageLimit group observer which follows the same pattern as
other UsageGroups. This specific observer allows the launcher to query
for the AppUsageLimit, available via the new LauncherApps API below. The
observer can be registered and unregistered via the respective new APIs in
UsageStats.

LauncherApps has a new API which allows it to get the AppUsageLimit for
a specified package and user, initally set via the API in UsageStats.
This new API allows the launcher to query specifics about the limit such
as how much usage time the limit has, and how much total usage time is
remaining.

Bug: 117409586
Test: atest FrameworksServicesTests:AppTimeLimitControllerTests
Test: atest android.app.usage.cts.UsageStatsTest#testObserveUsagePermissionForRegisterObserver
Test: atest android.app.usage.cts.UsageStatsTest#testObserveUsagePermissionForUnregisterObserver
Test: manual (mmma frameworks/base/tests/UsageStatsTest/)
Change-Id: Ifaffab629409e9191e40404a949c8df70bd3f7cb