Add onStart handler to AppDelegate

[dfabulich]

On Android, onStart is called right before onResume if the app is launching, or if the app had been stopped. On iOS, we're tracking this with a state variable.

Skip Pull Request Checklist:

• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
• OPTIONAL: I have tested my change on an Android emulator or device

---

• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

---