Allow DO to access DevicePolicyManager.isDeviceManaged()

This CL makes DPM.isDeviceManaged() accessible to the DO so that it
can be CTS-tested.

Bug: 32692748
Test: Device policy manager unit test + CTS & GTS in separate CLs

Change-Id: I5326e86b0ffee81d04bd48f0267044463a899b78