Here are some hard truths that every hobbyist Android developer needs to face at some point:
1. You are not going to make the next Flappy Bird - I mean seriously, I know that everyone thinks about their golden ticket. I would be lying if it didn't motivate me sometimes, but seriously, take a moment and scroll way way way way way down to the bottom of a search in the Play Store. The app you design is very likely going to be lower than that in search rankings. The likelihood of becoming a viral success is infinitesimal.
2. Your app idea isn't as good as you think it is - Yes, I know your mom (wife/best friend/whoever) told you that it is brilliant. But these are also the people who tell you how adorable your baby looks even though you know it uncannily resembles Yoda. It is hard to get honest criticism from the people closest to you, and it is hard for you, as the developer, to critically assess what you have made. But here is the thing, even if your app is awesome, you still have to contend with number 3.
3. Someone else (probably ten someone elses) have already made the app you are thinking of - Again, do a deep dive into the Play Store and take a look at what is available. I won't go so far as to say that there is nothing new under the sun, but people have been designing applications for Android phones for a long time now, and it is highly unlikely that you managed to come up with a totally original idea. And even if you did...
4. Marketing is as important as actual app quality - Do you have any idea how much SuperCell spends in advertising? A quick Googling says something like 500 million dollars a year. If you dig into the details, it averages out to a cost of nearly $2 per install if you use a CPI (cost per install) marketing campaign for your Android app. Now, to justify that kind of spending, you have to be able to earn more than $2 per user on average. Can your app do that? If not, you have to look into other ways of marketing your app. Let me tell you from personal experience, marketing is tedious. I started Android development because I wanted to make apps, not beg people to look at them.
5. You are going to get bad reviews - So let's say you overcome all of these obstacles and people are downloading your app. Certain people are going to leave horrible reviews for no apparent reason. Out of six apps that I have have made, I am lucky enough that one of them gets a slow but steady stream of downloads. The problem is that it is a utility app (it allows you to use your phone and a Bluetooth speaker like a megaphone) and I get one star reviews without comment and one star reviews with walls of text criticizing me as a person and one star reviews saying "I don't like this game, it is stupid", and it drives me up the wall. I know that all the other apps in the market face the same issue (the average rating of similar apps is like 3.5, even though they all work generally fine), but it is frustrating that people are apparently writing bad reviews not based on the app itself.
At any rate, Android development can be fun and rewarding, but it isn't the free pass to early retirement that you wish it would be. You need to have thick skin, and you will be surprised how you feel when you release your perfect app, check the console the following day and see that zero people installed it. It is deflating, but that isn't a reason to stop. Keep working on your ideas, keep trying new things. Who knows, you actually might create the next Flappy Bird.