Implement "flexible hybrid" split mode

Adds a new mode, called "flexible hybrid" split, that combines flexible split with legacy split, for a total of 5 split targets. Still has only 3 targets on tablets.

Bug: 349828130
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: TBD
Change-Id: I8e5c6b45e84a3d3d7abcd69384678f0dd1b0b7c6