Polish maximize menu

Use |Point| instead of |PointF| for maximize menu's position on the
screen.
Change Immersive button's width following ag/31915151.
Make Immersive button horizontally centered.
Merge |marginStart| and |marginEnd| into |marginHorizontal|.

Fix: 401210893
Flag: com.android.window.flags.enable_desktop_windowing_mode
Change-Id: Idc820c18793abbd8e2140a97891c2bf81ea1c9d0