Fix the HIDE_NON_SYSTEM_OVERLAY_WINDOWS permission definition

Change Ic51ead670fc480e549512ba1d02f49d9c13bc3f0 incorrectly tries making
this permission hidden.
While marking it with @hide and only adding it to api/system-current.txt
was the proper way on Android 6.0, on 4.4, we need to add system APIs to
api/current.txt, without hiding them.

This change adapts this permission definition for 4.4.

Change-Id: Ib2de017d396aa9dcefa91f7eb1ea210be668234d