C++ JNI layer naming convention

While working on the [JNI layer for Android's certificate verification](https://github.com/envoyproxy/envoy-mobile/pull/), I've noticed that there isn't a clear naming convention for the functions wrapping the C++ => Java JNI calls (note: there might be and I might simply be missing it).

As discussed [in the PR](https://github.com/envoyproxy/envoy-mobile/pull/2251/files#r867007275), I'm filing this to discuss/figure out one (it might also be worth to use a library to generate these JNI wrappers).