RootCanal: Execute AddHciConnection from the async manager context

AddHciConnection is called from the FdWatcher thread,
this causes a data race for accessing the TestModel state
when actions scheduled on the event thread get executed at
the same time.

Test: m root-canal && ./root-canal
Bug: 278891939
Change-Id: I8d59dca134a2a10e85073d1b7d6ed8c2e41e783c