Modify getPosition of disabled PointerController

PointerChoreographer has to combine fake finger coordinates with
the position of MousePointerController to process some touchpad
events.
The legacy PointerController which is disabled should return
(0, 0) instead of (nan, nan) on getPosition, so that
GestureConverter can set x-y axes correctly.

Test: atest libinputservice_test
Bug: 293587049
Change-Id: I7630cd16defc9324970f0c29851c80f1b9943977