Prepare for Docker build and run.

Enables running the `connectedAndroidTest` task inside a <a href='http://www.docker.com'>Docker</a> container to check installability of the app. This works in Jenkins too, e.g. <a href='http://www.griffenfeld.dk:8080/job/LockscreenDisablerTest/27/console'>this build</a>.

To use, install docker and run the following:

    docker build -t android-lockscreen-disabler .
    docker run --rm android-lockscreen-disabler

When you update your code, you can run the above again.

The benefits of running the emulator in Docker are reproducibility (the emulator will be clean each time) and portability.