Show bg color during ActivityEmbedding change anim with Shell transition

When switching between horizontal split and vertical split, it will
leave part of the screen empty. Show background color to cover that.

Also fix the leash start position.

Bug: 241043533
Bug: 254090083
Test: atest WmTests:TransitionTests#testChangeSetBackgroundColor
Test: verify with horizontal demo app
Change-Id: I125db07a631b19bd33bcff9935fe39f263cbb263