Avoid use of any in TelemetryListener

[robertbrignull]

In this PR:

• Use string index notation to access the private field instead of casting to any. As far as I can tell this is the "intended way" to access private fields.
• Uses the EnvelopeTelemetry type since it exists, instead of any.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.