Replace decorator pattern used in DoubleNumberLineAccordionBox

[jessegreenberg]

From phetsims/sun#860, components with "content" cannot be decorated with other Nodes and this case was found.

unit-rates/js/common/view/DoubleNumberLineAccordionBox.ts

Lines 145 to 148 in 24ae5e0

	// Adjust rectangle to match accordion box size.
	thisBoundsNode.setRectBounds( this.localBounds );
	this.addChild( thisBoundsNode );
	thisBoundsNode.moveToBack();

A potential way to replace this is in the following commit, @pixelzoom can you please review and change if there is a better way for this sim?

[pixelzoom]

Thanks @jessegreenberg, that's a nice improvement. Closing.