libprotobinder: Add driver stub and unit tests

Added BinderDriverInterface to allow hooking of
driver calls.

BinderDriver provides access to the real driver.
BinderDriverStub is a driver simulator that can
be used for testing

Also added unit tests for BinderManager, BinderProxy
and BinderHost

BUG=brillo:582
TEST=new unit tests, ran psyche_demo on gizmo

Change-Id: I986f5c61e028f1ef7fd55c9d89f6c45c677ad7e0
Reviewed-on: https://chromium-review.googlesource.com/266460
Reviewed-by: Lee Campbell <leecam@chromium.org>
Commit-Queue: Lee Campbell <leecam@chromium.org>
Trybot-Ready: Lee Campbell <leecam@chromium.org>
Tested-by: Lee Campbell <leecam@chromium.org>
8 files changed