Removed unneeded search for root token in WindowState ctor.

The search will always return the WindowToken used to give the key to
the map since there is a one-to-one key-value mapping of IBinder token
to WindowToken that is enforced in WindowToken ctor.

Bug: 30060889
Change-Id: I732ec7da5872ed22c7832690ea6a2cebc9b492f8