Flexible Split: Reference class

Adds SplitSpec, a class that stores the bounds of all valid split layouts (recalculated whenever config changes).

These bounds can be accessed through SplitState, and are intended for general use in flexible split (as a single source of truth), though they are currently only calculated and not used.

Bug: 349828130
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: Only generates class with flag on. No effect on behavior, either with flag on or off.
Change-Id: I1bd3dd6221b179c5d42cc7bb0e674070c4136e4c