Hi there,
This was an easy one.

Tools used: apktool, grep (yes, that's all)

First I installed the application on an isolated device without internet connection and ran it.
It's just a makeover of the game Flappy Bird with UUTCTF logo in the background.
Running the game for a few times things I observed were:

Hint 1: When the game started, you see an odd string at the place of scoreboard (typing in words since unable to type the chars) dollar underscore dollar(without spaces).

That's all.

Go to your terminal and

apktool d FlaggyBird.apk

You will get the decompiled app in a directory.
Then you just grep through all the files for the string in the hint, and you will find the flag in chunks of 3 letters in a file,
I grepped for uut before that and the things were more easy for me.
```/smali/com/arjundass/flappybird/a.smali```
UUT-CTF:Kn3ll_4_M4sT3r_XD--

Flag: UUTCTF{Kn3ll_4_M4sT3r_XD--}

take the flag from the word put it in the flag format, and voila :))