31028 java desktop build

This PR splits the project into 2 jars, one core and one Android-specific.

The build is achieved with 2 gradle sub-projects, which might make importing into Android Studio a bit more tricky.

The android project currently only contains com.cloudant.sync.sqlite.android.AndroidSQLite and com.cloudant.sync.sqlite.android.AndroidSQLiteCursor. Also introduced is a bit of dynamic class-loading (Class.forName), for loading the right SQLDatabase implementation and dynamically fetching Android run-time info without making a hard dependency on these classes.

Also updated are the README and sample project, which has been successfully tested on the emulator.
This ends up in the generated POM, but isn't required for the Android build. I think we might be able to use the "runtime" (or similar) dependency to remove this from the POM without needing a separate `-j2se` package which essentially just depends on this and has the SQLite wrapper code.

However: how much work would it be to split out this into a separate jar?