I have seen bounty hunter doing well, I have also seen people who do not participate in bounty and are making it well with trading alone, but I have never seen anyone around me being involved in mining, I learnt mining is more risky and cannot just be done alone, and if you even attempt it probably with your android, you may end up wasting your time and not even making it as the traders and hunters. It tend to take a massive effort to be successful in mining, you need good electricity,good mining wallet, computer with good capacity, and some other gadgets. Considering all this, which of all the participation do you see as least demanding and easiest?
As you already said it, for mining you need to start with a lot of investment first, you need some equipment to start off and think about it even if you do manage to mine a few blocks that probably won't earn you much when you're up against pros who can invest in the largest computers CPUs etc.
Thus, the only two viable options without prior investment is bounty hunter and trading. The trouble with bounty hunters is that you mainly get paid in tokens and so you're not actually 'earning', only if you get lucky the token might get listed but this is not the norm.
With trading, on the other hand, you can start but then again you need to have some money and some knowledge in order to make profits, and it is risky if you're not sure about how to interpret the market volatility.
These are some of the pros + cons for the options available. Only you can decide for yourself which one suits you best.
Hope this helps.