Reduce flicker when resize animation

When resize split, split decor use fade in animation but bottom
larger than bounds area is black, it will make color change from
black to decor background color and it just like flicker.

To avoid this, we should add back using of am#setSplitScreenResizing
which could make black backgound to color similar to app content
background. And it should also reduce flicker while snap divider to
dismiss split.

Bug: 220985951
Bug: 221178408
Test: manual
Test: pass existing tests
Change-Id: I961a4d3ab4f3552fd5ace4b3a6742afa980f073c