ngclient: Make updater.config immutable

This comes from #2363 / #2359 : we should make the config somehow immutable once it's been used by Updater. This should be a difficult mistake to make:

updater = Updater(...) 
updater.refresh()
updater.config.offline = True

(the issue is refresh() already used config.offline: the value can't be changed during lifetime of Updater. Likely this makes sense for all config)

I originally suggested reading the updater.config.offline value only once during initialization, but that still makes writing the above code possible. Would be better if it failed linting.

I suppose two possible actions:

• Updater.config becomes a read only property (or not public at all)
• Could make UpdaterConfig frozen=True as well

まず、issue の例を使って設定がいつ読み取られるかを特定しながら、refresh() を通じて Updater.config と UpdaterConfig を追跡します。読み取り専用プロパティと凍結された設定のオプションを比較し、その後、Updater が設定を使用した後に設定を変更できないこと、および既存の動作が引き続きカバーされていることを確認します。