don't expose main thread message loop

ensure all calls go through the actual API

(will back message loop thread with rust's tokio runtime if running
with rust, to reduce thread transfer overhead at shim boundary)

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run
Change-Id: I4d55a4b35a620d63638004cb0e553bbcc04dd69b