Fix setting backdoor port in service start

Previously if self.manager wasn't defined for a Service object but a
backdoor_port was specified then that particular service would not
start because it would try to set the backdoor port in an object that
didn't exist. This commit fixes setting the backdoor port only if
manager is defined when calling start().

Closes-Bug: #1243260

Change-Id: Ib0e709cfd0e299a3e010450b7eba6e92bc108e4e
Merge "Fix setting backdoor port in service start"