(2/n) Implement BackgroundWindowManager to manage tutorial background

Using WindowlessWindowManager for adding a surface wihtout depend on
Window Hierarchy, we just add leash with specific theme color at
bottom z-order while One-handed trigger, and remove it when exited.

Bug: 197165590

Test: manually test One-handed mode basic operations.
Test: atest WMShellUnitTests
Change-Id: Ie53684edd4511719516c00dd64c2dfad360a593b