Factor vertical split into menu position calculation.

Currently, the calculation of handle and manage windows menus is only
accurate for left-right split, not top-bottom split. This CL applies
bottom stage location when calculating desktop menu's position for
bottom stage tasks. Also migrates handle and manage windows menu
position calculation into a utility class.

Bug: 380879525
Test: Manual
Flag: EXEMPT bugfix
Change-Id: I8c4c09103c2f0bd72fdc1b26e0e6c365b3596159