Don't apply overscan insets to freeform windows.

Overscan gets applied when window frame is outside of the display frame.
This is a natural situation for freeform windows, so they should not
receive the overscan inset. Applying overscan insets causes the
application content to get squashed, when we resize the freeform window
and part of it is outside of the display.

Bug: 26009343
Change-Id: Ib614fb8c7170c3b6d69614b813674853e1204eef