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