Integrate Android build procedure.

* .travis.yml updated to download the Android SDK and NDK, and build
  the Android app with `ant`.
* The src/app/JNI directory has a basic `Android.mk` makefile that
  builds the (currently) empty `drops-jni.c` just to make sure the
  toolchain works.
* `build.xml` is the `ant` build file.

The build fails because of #10