Made WindowState.mChildWindow private scoped

This involved:
- Moving method that operated on mChildWindows and mostly touched
WindowState fields into WindowState class.
- Creating new methods for getting information about child windows
outside WindowState class instead of accessing the child list directly.
- And, tests ;)

Bug: 30060889
Change-Id: I339cecb926b44b5db7ead1970e356304d5429c8f