Without seeing your xml, the only way you will be able to accomplish this, is with is with a single relative layout, that contains another LinearLayout containing your row of buttons.
While you are correct, that it is good practice to limit the number of nested layouts (not just linear, but nested layouts of any type).
In this case, to get your row background set, you will need at least a single nested LinearLayout, and this is not a big issue.
Edit: After reviewing your XML, I do think this is the best way to achieve what you want.