RootCanal: Add address prefix parameter to TestModel()

The parameter can be used to configure the prefix for all
Bluetooth addresses generated from a RootCanal instance, so as
to disambiguate addresses generated from multiple connected
instances.

Bug: 254735655
Test: m root-canal + presubmit
Change-Id: Idda6c6dc56f5e56099df9079724587b3bef2193e