Allow DO to provision even if it has set disallow remove mp.

If the device owner has set DISALLOW_REMOVE_MANAGED_PROFILE,
and there is already a managed profile:
it should be allowed to provision a new managed profile by
deleting the old one.
Test: adb shell am instrument -e class
com.android.server.devicepolicy.DevicePolicyManagerTest
-w
com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
BUG:34116228

Change-Id: I9e6f39924107aee40b57d22e638487a1ea3132de