Add limits to App Usage Observer Api

Added a minimum time limit for App Time Limit Observers and a maximum
number of observers an app can register to AppTimeLimitController

Also fix small bug with mDpmInternal initialization

Change-Id: I8c0861d79711149f3ecd92ba4660f7b207e9b921
Fixes: 78890515
Fixes: 79230583
Test: atest AppTimeLimitController
Test: atest cts/hostsidetests/devicepolicy/src/com/android/cts/devicepolicy/ProfileOwnerTest.java#testAppUsageObserver