Clear LastSurfacePosition when surface is cleared.

This is important for restoring freeform window from recents because
it's possible that the surface is removed but the last surface position
is still there, which stops the surface position being updated later
when the task is restored.

Bug: None
Test: 1) Launch an activity on default display;
2) Close the activity by pressing back button;
3) Go to recents;
4) Restore the activity into freeform by long pressing the icon and
select freeform;
5) The activity is shown at right location rather than (0, 0).
Test: atest WindowContainerTests

Change-Id: Ia5e28101b2343c2fe189da0a73351c72c32df5d3