Use abstractions to allow FullscreenUserSwitcher take advantage of SystemUIOverlayWindow.

This change includes the following:
* Rename SystemUIPrimaryWindow to SystemUIOverlayWindow
* Create Mediator and View controller abstractions that allows
developers to easily take advantage of SystemUIOverlayWindow that is
managed by a single SystemUI Object: SystemUIOverlayWindowManager.
* Convert FullscreenUserSwitcher to take advantage of the newly added
abstractions.

Bug: 147826738
Test: Manual
Change-Id: I19825da81f8d1b1259a2ba115e0238a9ffa69e37
Merged-In: I19825da81f8d1b1259a2ba115e0238a9ffa69e37