TrigDecisionCnv: use xAODConfigSvc for accessing HLT chains

Use the `xAODConfigSvc`, which gives access to the trigger menu
converted into Run-3 (JSON) format, in the `TrigDecisionCvnTool`.
Make the config service configurable as a property and setup the
`xAODConfigSvc` in `TriggerConfigGetter.py`.

Apply some code cleanups in passing.

Relates to ATR-23580.