Refactor the OnesPlayAreaNode

[Luisav1]

From #75

Are there any TODO or FIXME or REVIEW comments in the code? They should be addressed or promoted to GitHub issues.

The parts of CountingGameLevelNode.js, and other screens that are used for the play area need to be refactored once the play area is updated. Once the bounds handling for the OnesPlayArea and the paper numbers are improved, we don't need to add a y margin to them anymore.

[chrisklus]

This work was completed in #118, closing.

[zepumph]

One more TODO pointing to this issue:

number-play/js/game/view/CountingGameLevelNode.ts

Lines 61 to 65 in 4404f84

	//TODO https://github.com/phetsims/number-play/issues/82 The parts of this file that are used for the countingArea node need to be refactored once the countingArea is updated.
	const countingAreaNode = new Rectangle( {
	rectWidth: RECTANGLE_WIDTH,
	rectHeight: RECTANGLE_HEIGHT
	} );

[chrisklus]

Last TODO improved above, closing.