For #153, return to duplicating the "net" png in the absence of extra graphics, because selectors don't seem to work.

This will probably stop DevDrawer crashing too.

A sufficiently smart build script could notice that when I do have drawable/foo and drawable-v7/foo, and a min API of 7, it could perhaps omit drawable/foo from the package. I don't know if the actual build scripts are that smart.