Code review comment for lp:~kdub/mir/refix-1517205

I'm wasn't too flummoxed by mirplatform being used by the client, but OTOH, may as well not link to it if we can help it.

I sidestepped the problem by pulling the new code from mirplatform to src/platforms/android/common. (a directory for common-to-client-and-server android code).

I also bumped server ABI for the 0.20 series, as we have indeed broken the server abi (but not api) for those using mg::Buffer's