Kotlin-ify MagneticDividerUtils(-Tests)

- Make MagneticDividerUtils an object
- Use the MotionBuilderContext as a scope for the generateMotionSpec
- Convert test to Kotlin

This is done to (a) reduce nesting and thus increase readability, and
(b) to facilitate writing tests in the next CL. It removes the need to work around a device specific Resources object

This is a pure mechanic refactoring

Test: MagneticDividerUtilsTests
Bug: 383631946
Flag: com.android.wm.shell.enable_magnetic_split_divider
Change-Id: I0684d1e9d5db5125d6a47a4f1fecfef8c5e9c5c7