Reset side stage position after child task vanishes

This CL resets side stage position in split screen when all stages has no child tasks. This ensures the new split screen can be opened from the default position.

Bug: 333270112
Test: open links from Gmail
Flag: EXEMPT bugfix
Change-Id: I9c512f7dcf822b05fee0df1a06a3f3dd17ff3d24
1 file changed