building with gradle-experimental, and it should be using CMake

[gkasten]

Details at "Header files not found trying to build for Android" https://groups.google.com/forum/#!topic/walt-discuss/ikdJ83ZBDDM

[ggfan]

the latest in the repo should be able to build. https://github.com/google/walt/pull/115 would be the final touch up. The strategy:

• reuse the existing android.mk with Gradle's ndkBuild plugin
• application.mk is not used, intended to set up those variables inside app/build.gradle if needed in the future.