fix: returns simple acitvity name instead of full class name

[marandaneto]

:bulb: Motivation and Context

Follow up https://github.com/PostHog/posthog-android/pull/153 If there's no label on the activity and app, the class name is returned with the full package name.

:green_heart: How did you test it?

:pencil: Checklist

• [X] I reviewed the submitted code.
• [ ] I added tests to verify the changes.
• [ ] I updated the docs if needed.
• [X] No breaking change or entry added to the changelog.

[marandaneto]

cc @thisames