Improve ergonomics of calling Java from C++

[jfirebaugh]

To call a Java method from C++, you currently must do quite a lot. As an example, consider Number::floatValue. You must:

• Define class Float with a Name method returning "java/lang/Float"
• Declare and initialize a jni::Class<Float>
• Call GetMethod<jni::jfloat ()>(env, "floatValue") on it
• Once you have an Object<Number> instance, do number.Call(env, method)

Ideally, you'd be able to just do number.floatValue(). Is there any way we can get closer to that?

Refs mapbox/mapbox-gl-native#8809