Crop to stack bounds during animation if docked task is non-resizeable

When a non-resizeable task is docked, it's running in fullscreen size, its
visible area should always be cropped to docked stack size. Otherwise wrong
size might show through during animation.

bug: 26163416
Change-Id: Idcbf4b86ae0976d86a2f91e78ccc2113dd86af62