Replaced use of WT.appWindowToken with WT.asAppWindowToken

Not a perfect implementation, but slightly less confusing as
the child class is no longer a member variable of the parent
class.

Also renamed WindowToken.service to WindowToken.mService to be
consistent with how we name in other places.

Bug: 30060889
Change-Id: Ib21472a784a4f24e7789d443876cf0912d9e89de