SystemUI: Allow privileged system apps to access screenshot service

This is necessary in order to take a screenshot from other apps using
platform APIs as if the user used the screenshot keychord, which we need
for implementing gestures in a modular way. The service is still
protected by the INTERACT_ACROSS_USERS_FULL permission, which is already
necessary to get far enough to attempt binding to this service.

Note that existing screenshot APIs are not sufficient because they only
allow apps to get an image of the screen; we want the full user
experience of the animations, actions, preview, sound, etc. that
normally come with a user-initiated screenshot.

Change-Id: Ieb559d5dc921393f68aa1d6142f7834d89d4e22b