Fixes a regression from 3f94708eff. ExecutableFinder returns false if
the executable isn't found, but CommandFactory was expecting a null response,
This caused autodetection to always think firejail was present.
Adjust CommandFactoryTest to ensure we're always passing a string to
FirejailCommand. We need to switch findFirejail to protected so it can
be mocked.
Bug: T257282
Change-Id: Ie73418ebef6dce2bd5ec18fa38e29219d5bb2fd6