MonitoringCertTask no longer relies on software.device_admin

Added a test to validate that it still works the way it should before
and after the change.

Bug: 33258404
Bug: 35196414
Fix: 35129745
Test: runtest -x services/tests/servicestests/src/com/android/server/devicepolicy/DevicePolicyManagerTest.java
Test: also manual, instructions:
Test: (1) Disable software.device_admin from tablet_core_hardware, rebuild.
Test: (2) Install CA cert. Notification should appear.
Test: (3) Reboot. Notification should still be there.
Change-Id: Id992725c1844a2fffbde4d8acaba531e99f853ad