Rename WindowState.mAttachedWindow to WindowState.mParentWindow

Helps with code clarity and reduces confusion for upcoming WM
object modeling changes.
Also, along the same lines changes locations that check to see if
WS.mAttachedWindow wasn't null to determine if the window is a child
window to use WS.isChildWindow method instead.

Bug: 30060889
Change-Id: I9b975ab9ff9bf09cdd3c0dcaddd3bf9232e88be8