If i turn off root permission i can't login with gmail with the unsigned. More security = bye bye mod.
If we're talking only about Brown Dust 2 then you dont really need an unsigned apk.
1)download clean version from the store
2)login with your google account,when the "download resources" thing pops up you close the game and delete it
3)install the game with signed apk,login into your google account
4)download resources,enjoy