Enable mouse hovering on sides to show system bars

Before this CL, mouse hovering events can only show sytem bars from top
or bottom. However, we might have system bars on the left or right. This
CL enables mouse hovering events to show system bars from any sides.

Bug: 234093736
Test: Connect a mouse and see if hovering events can show transient bars
      from each side.
Change-Id: Ida8d01534db57f51a6720f9c9f28e886ac3311ad