[Grid] Enable the Grid helper in Compose using JSON representation

[jswong65]

Enable the Grid Helper in Compose using JSON representation.

• Moves the whole logic of Grid to core (GridCore.java)
• Adds a reference object for Grid (GridReference.kt)
• Updates Stata.java and ConstraintSetParser.java to parse Grid helper and create a reference object proper.
• Adds ConstraintLayout and MotionLayout demos (GridDemo.kt, MotionGridDemo.kt)
• Adds tests with ConstraintLayout and MotionLayout (GridTest.kt, MotionGridTest.kt)
• Adds corresponding demos for the tests to better visualize the tests (GridTestDemo.kt, MotionGridDemo.kt)

Todo: Test setters when working on DSL.

Sample Demos: GridDemo1:

GridDemo6

Sample Motions: MotionGridDemo

MotionGridDemo2