Splitscreen: Use MotionValue to implement magnetic snap zones

First pass at creating magnetic snap behavior for the divider, behind flag.

Bug: 383631946
Flag: com.android.wm.shell.enable_magnetic_split_divider
Test: Divider now snaps while the user is dragging it.
Change-Id: I431aa8d3b052c90f3d1a753dd181a129f0b401ca