Don't freeze the screen when resizing task or stack.

The freeze stops both the input to come and the drawing, which makes
the resize look like it's lagging. We use the display freezing to
prevent artifacts when using big surface for resizing. However,
freezing the whole display is too aggressive. We need subtler
approach.

Bug: 24575766
Change-Id: I8799e7989e937101129572142dfe6b28a7ed7692