Change divider ratio for flex to use display width instead of right app's right bounds.

* BottomRight app's right bound is now offscreen, whereas for the
divider fraction we want the fraction of what is visible on-screen.

Test: Logged reisizing ratios to see the correct value
Bug: 392131500
Flag: com.android.wm.shell.enable_flexible_two_app_split
Change-Id: I0840153983e29b4aa4d131d2f8dbd2f2fb756aa0