Synchronize results from runner thread to main full backup thread.

Previously, all results from runner thread - both for preflight
or full backup pass were ignored.
This change adds two synchronized method to get preflight result
and result of full backup pass.
This leads to better coverage of AGENT_ERROR to return in callback
as a result of backup operation.
On the other side we won't start backup pass in main thread
if preflight check hasn't been succeeded.

Change-Id: Id5f9e4c956a1bd5c396d59b7ad2098139a15e69d