android_gradle no longer builds

[paulthomson]

The Android app under android_gradle/ no longer builds because the jcenter and bintray repositories were shut down.

I have a fix. We can essentially take the latest sample Android app project and update our build files to match. While we are here, there are a few other things we can improve. E.g. we can have gradle automatically build the native code via CMake, rather than having to manually build via ndk-build and copy in. We may even be able to eventually remove the Android.mk files, as I don't think they are used.