[wm] Use existing parent window token for child windows

The child windows should be attached to the parent window rather than its window token.
The window token can be visible state only if it has child containers.
So, if the window token is created only for the child window, then it cannot be visible state.

Fixes: 168713890
Test: atest WmTests:WindowTokenTests
Change-Id: I3efa14fda7a5674a178b13ff6310a526e75e3b50
Merged-In: I3efa14fda7a5674a178b13ff6310a526e75e3b50
(cherry picked from commit 347196a3)