Remove mTask from AppWindowToken.

mTask was a duplicate reference to the WindowContainer's parent. The
main benefit of this refenrece was type, as its callers require an
instance of type Task.

With the introduction of the method getTask, we can shift away from
this variable and instead directly cast the parent here. If the
window hierarchy changes in the future (such as AppWindowToken parent
types not being Task), we can re-evaluate and adjust how task is
returned.

Test: manual
Change-Id: Idd6bba1121056ed79745911efe838edfa685bbf2