fix(installer): make isDownloaded robust + unify parameter naming

[joshtrichards]

• Related: Consolidate app management code #8505

Summary

• Check for presence of info.xml in app folder, not just the presence of an app folder (which could be empty/incomplete/etc)
• Unify parameter names for the "appId" for all functions in the class

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)