Magnetic target library!

This adds the core library and tests. See design doc at go/magnetic-target-design. Upcoming CLs add animations for the target enveloping the object, and support for horizontally flinging towards targets (it currently assumes top/bottom targets).

Test: atest SystemUITests
Bug: 138116784
Change-Id: Icf5b3bc0359e53c137718738e6b800b7bca04770