Fix DPM check logic for AppUsageLimitObserver (un)registering

In the case that the DevicePolicyManagerInternal Local service does not
exist on device, the permission check for
(un)registerAppUsageLimitObserver should not be skipped.

Test: atest android.app.usage.cts.UsageStatsTest (must be on a device
with <unavailable-feature name="android.software.device_admin" /> in
it's features xml)
Fixes: 155360727

Change-Id: I8a88e03a7636956d27f531cf989bda1a32799f07