Create public API for policy transparency outside of Settings

The new DPM.createAdminSupportIntent() returns an intent that shows the
"This action was disabled by your admin"-dialog from settings.
This enables apps to inform the user about the cause of restricted
functionality.

A new extra for the intent allows to specialize the dialog for different
restricted features, instead of a generic message for all features.

Bug: 31215663
Test: runtest -c com.android.server.devicepolicy.DevicePolicyManagerTest frameworks-services

Change-Id: I3de7aeec0f88b8f013a63957aec803cd123fbedc