Flexible 2-app split: Pinned Taskbar

Adds the ability for DividerSnapAlgorithm to recalculate when Taskbar is pinned or unpinned. The divider will animate appropriately, if needed.

Also cleans up and adds a specific SNAP_MODE for flexible split (no longer shares a SNAP_MODE with SNAP_FIXED_RATIO).

Bug: 349828130
Test: When flag is off, behavior is unchanged and continues to pass tests
Flag: com.android.wm.shell.enable_flexible_two_app_split
Change-Id: I1a3276b23acd7c2d22df1388004abd3a60a6d10f