This is called Android Reverse-Engineering.
There are a lot of tools out there that could decompile your apk or any apk in seconds.
An example is APK Studio which even comes with cross-platform support.
There are various IDEs for decompiling/editing & then recompiling of android application binaries.
There are even ones that completely are re-written using QT for cross-platform support. This will permit you to have multiple frameworks installed & to pick a particular one on a per project basis.
A one solution to prevent Android Reverse-Engineering is to just get patents for your code, and it will save you from a lot of trouble.