Fix Gradle build and add build info to README.md

Fix Gradle build: The previous Gradle commit only
worked due to my very specific setup and device
state. It should now work with a clean setup as
well.

Readme: Add a section on how to build the project
with Gradle to README.md.

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Sorry about the last pull request not working for everyone. It looks like there where some files cached on my device and build directories.
But as soon as I cleaned my build files again and/or removed the app from the device it would no longer run as the native lib was not included in the build.
Also some .jar libraries where included twice and caused duplicate symbols.