WindowList be gone!

The use of DisplayContent.mWindow list to track all windows is
no longer needed as we can now get windows through the window
container hierarchy with methods like forAllWindows. The window
list was also a very complicated logic to understand and maintain,
so it won't be missed :)

Bug: 30060889
Test: Existing tests pass
Change-Id: I590cb33aa0f42bcd4a26ddce102f05e657f54144