If you want to right away try the demo, then you can directly download the nightly build from this https://ci.tensorflow.org/view/Nightly/job/nightly-android/. Go to Last Successful Artifacts then go to the out folder to find the tensorflow_demo.apk file. You can Also drop use it as a precompiled native library into your own applications.
Look at the following URL for more details:
https://github.com/tensorflow/tensorflow/blob/master/tensorflow/contrib/android/README.md