Fix Qt bootstrap handling of non-back key events

[lazycat11]

Summary

• handle only Android Back key events in the Qt bootstrap double-tap-to-exit logic
• pass all other key events through to Qt via super.onKeyDown
• checked the other bootstraps: webview/service_only already guard KEYCODE_BACK, and the remaining bootstraps do not use this double-back exit logic

Testing

• git diff --check
• fork GitHub Actions workflow Unit tests & build apps run 26929233140 completed successfully, including Flake8, Java Spotless, pytest matrix, recipe tests, emulator run, and Qt app build

[kuzeyron]

This seems not to be the best approach as it's colliding with the rest of its code and its purpose. Instead of doing this you should try another approach from the app itself on how it handles the back key.