Executando o desafio Extra Credit:
Change the background color when the button is tapped

Add an ID to the RelativeLayout element in activity_main.xml. Then you
can use that ID to get a RelativeLayout object just like we get a
TextView and Button. The RelativeLayout object has a method called
setBackgroundColor() that takes an int color code as a parameter. Use a
color code from the Color class, i.e. Color.GREEN, and call this method
when the button is clicked.