Replace DisplayContent#OverlayContaines with OverlaySC

(2/n DisplayArea Sibling Roots)

Partially revert ag/9745181. We no longer need a window container for
overlay. After we moved magnification overlay to display area hierarchy,
the current overlay container is only used as a SurfaceControl, so we
move it back to be a SC.

Bug: 156425461
Test: atest WmTests:DisplayContentTests
Test: manual: verify fullscreen magnifation, screenshot, screen
rotation.

Change-Id: Ifa22b0d58b67aa5e385d4ca7d23afc96c89d15c3