Fix start timestamp not set in ActivityLifecycleIntegration

[adinauer]

#skip-changelog
Guess changelog is not needed since the bug hasn't been released yet

📜 Description

Messed up a refactoring

💡 Motivation and Context

Start timestamp was set after calling startTransaction so had no effect. This fixes it.

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

[bruno-garcia]

Test that validates the fix and avoid regression is advised

[adinauer]

Test that validates the fix and avoid regression is advised

There are tests that validate the timestamp has been set, the problem is, it doesn't care whether the timestamp was set before or after calling startTransaction as long as it's set at the end of the test (which it still was).