Document Android system bars behavior after immersive mode removal

[vishal2005025]

Fixes: #602

📝 Summary

This PR updates the Android SDK documentation in the Jitsi Handbook to clarify current system bars behavior after the removal of immersive mode in the Android app.

Recent versions of Jitsi Meet no longer use Android’s immersive full-screen mode, which previously hide the system status/navigation bars. Instead, UI behavior now follows Android OS defaults. This change was introduced in PR #16513.

📌 What’s included

A new subsection has been added to the Android SDK documentation:

• Explains that immersive mode was removed
• Notes that system bars are now fully controlled by the OS
• Advises SDK embedders to respect safe insets and avoid placing UI under system bars
• Recommends testing layouts across Android versions and device types

🎯 Why this is needed

The change in UI behavior affects:

• App layout expectations for developers embedding the SDK
• How the app appears in fullscreen scenarios
• Consistency across Android devices and newer versions (Android 14/15+)

Documenting this behavior reduces confusion and helps integrators design properly around Android system UI constraints.

🔗 References

• Removal of immersive mode: dep(react-native-immersive-mode): remove everything related to this jitsi-meet#16513

[vishal2005025]

@damencho please take a look.