Fix issue where 70:30 ratio was showing up outside the flag

In ag/33362389, the introduction of flexible "hybrid" split (with both 70:30 and 90:10 snap ratios available at the same time) unintentionally caused a bug outside of the flexible_two_app_split flag.

In some display sizes, the lack of screen space causes the 70:30 ratio to be disabled. ag/33362389 caused the 70:30 ratio to show up even in these scenarios.

This CL fixes the logic that caused that error, and also attempts to clean up and standardize all of the logic paths used in determining split layouts.

Fixes: 422912855
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: Manual repro with affected devices
Change-Id: I3e94b96fab8899de4672a86ce2cec2c54e9d6a54