[omaha client] Implement single request state machine

This is the start of the central state machine which makes requests to
omaha.  This is the initial skeleton if the async flow of performing a
single update check (and subsequent update if one is returned by Omaha).

Bug: PKG-465
Test: fx run-test omaha_client_lib_tests
Test: ran prototype client (not in this CL) with Omaha service

Change-Id: Ic6e07f76bd30f903ee232f61a20ca1f595adeb74