Fix NPE when resizing with transferring focus failed

If InputManager.transferTouchFocus failed when resizing, the
TaskPositioningController will call TaskPositioner.unregister
to unregister InputChannel. But the mClientCallback used by
unregister is initialized by TaskPositioner.startDrag called after
InputManager.transferTouchFocus, so it will cause NPE problem.

Add NPE checking before using mClientCallback to fix this problem.

Test: atest WmTests:TaskPositioningControllerTests

Change-Id: I245a6a3591975cdbfa6453e490cb88aa2c2fe672
Signed-off-by: utzcoz <utzcoz@gmail.com>