Implement SET_PROFILE_OWNER intent

This will be shown when a system priv-app tries to add a profile owner
to a device that's already been through setupwizard.

Refactored the Add Device Admin dialog to also be used for this
purpose with additional warning text.

Also, make sure that profile owners cannot be deactivated.

Bug: 16207721
Change-Id: I25499a22718b2219a5a56b158ca2681243751549