Allow profile owners to set the unknown source setting

This allows work profile MDM to enable unknown sources
even if the user doesn't have UI for it. Installing an
app from an unknown source will still prompt the user
with the package installer dialog, so it's not like the
MDM can now quietly install apps from non-market sources.

Bug: 18316350

Change-Id: Ia8f4fe36f12a258aa888e085acc0b358925f4817