SysUI: Add method for plugins to keep status bar full screen

Lots of things detect overlays these days (installing apps) and the
only way to avoid the problems associated with this is to live in
the status bar window. So allow plugins to hold the window open
when they want to so they can have overlays be in that.

Move StatusBarWindowManager to Dependency to make things easier
as well.

Test: Install the example plugin, test can access QS and interact
      with apps

Change-Id: Ib2288bf56704960847217bad01a480ab407e0ffe