Fix issue #72151749: Lock down 'clear cache' functionality to...

...only system signature

Apps that hold the original permission are silently ignored.  There
is a new internal permission you must hold to actually delete cache
files.

Test: booted and run, can still clear cache in settings.
Bug: 72151749
Change-Id: I47a5cb697e1a4fa3665ac71aaabcda57bef7b561