Create JVM-only build target

Envoy Mobile has very few practical Android dependencies, and for the most part they're well isolated in distinct classes/files. By and large the library can run in theory on a pure JVM stack without Android libraries, but there's a couple additional abstractions needed to actually create the build target.

Allowing Envoy Mobile to run on a JVM without Android both makes it faster/easier to run a variety of tests, and allows for more use cases for the library.