Add policy transparency to disallow remove user.

On the Settings > System -> Multiple users screen, ensures that the"Delete from this device" menu is always available, but is disabled and includes policy information when DISABLE_REMOVE_USER is set.

Change-Id: Ia6c6cfb360f35a6e447bf9d85d2472ac11dde1ac
Fix: 113807450
Test: m ROBOTEST_FILTER=UserSettingsTest -j40 RunSettingsRoboTests; CTS Verifier Device Owner Tests/Policy transparency test/Disallow remove user.