Implement tooltip for split divider drag

A tooltip will show if the divider is dragged more than halfway toward a new snap position.

Known issues:
- On certain displays/orientations, the tooltip surface can be truncated because it exceeds the bounds of its parent.
- Strings, measurements, and colors are not finalized.
- No animations or rounded edges.

Bug: 349828130
Flag: com.android.wm.shell.enable_magnetic_split_divider
Test: Manual
Change-Id: I5cd355810c20bf2a553e56165ef51ac5849341df