I extracted my project apk file and found 2 "unknow" files named .classpath & .project. What the purpose of these files?
Ps: It only appears when I use B4A version 2.02.
These files are included inside one of the libraries that you use. They do not need to be in that library (they part of the Eclipse project), however they do not do any harm.
These files will be in the JAR file of one of the libraries that you are using.
When you compile they'll be added to the APK.
Unless you open the library JAR file and remove them there's no way to stop them being added to your APK when it is compiled.
I think you can use WinRAR to open a JAR and remove those files.