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There really aren't any games like that on Android. The closest (and I mean that in the loosest sense of the word) games are Dead Trigger and Dead Trigger 2. They're not open world, but they are zombie survival horror games with somewhat large or open levels, varying objectives, and upgradeable weapons. The first game in particular is less money-hungry than the second one (or at least it was when I first played it almost 10 years ago).
There is also Unkilled by the same devs, but from what I remember that game leans even more heavily toward pay-to-win than the first two games.
I should also mention Into the Dead. This game is basically a runner-type game with a first-person view, but it does have a cool atmosphere and you do get to use weapons to fight off the zombie horde that is trying to eat your brains. It's one of the few runners I like. There is a sequel as well.