Add unit tests for region box state class

This commit aims to write unit tests specifically aimed at testing and verifying the behavior of functions found within the region box state class. This includes the startDrag, drag, and endDrag functions. These tests are comprehensive and will furthermore test functionalities of code outside of the region box state class (but still directly related) such as functionalities found within the ResizeZone.kt for resize operations. Helper functions are made within the test file in order to reduce any redundancy while having full test coverage (such as testing all different corners/edges). Lastly, this commit also changes the visibility of several functions/variables/classes within the RegionBox to "public" in order to access/test it from the new RegionBoxStateTest file.

Bug: 436265991
Test: atest RegionBoxStateTest
Flag: TEST_ONLY
Change-Id: Id20750e506df1f38b91339663a8698da128f88ed