JNI cleanup/refactor

[tobrun]

We need to find a way to make our current jni.cpp implementation a bit more manageable.
Using jni.hpp has given us a modern type safe wrapper c++14 approach to the jni but it doesn't help us manage the amount of configuration we currently need to do. We should look into structuring the content differently or approach the idea in a different way.

cc @mapbox/android

[jfirebaugh]

• Most of jni.cpp is using the low-level wrappers. Switching to the high-level wrappers will cut down on a lot of boilerplate.
• Code in jni.cpp should be split out into individual files for each native type.