fix: windows, window size, fullscree & maximized

[fufesou]

In the default example, the window height is incorrect when maximized.

Reproduce steps

Windows -> Display settings -> Scale (any value other than 100%)

Win11, scale 150%, 2160x1440, window_manager as an example (same logic).

1. cd example && flutter run -d windows
2. Find the output like The Flutter DevTools debugger and profiler on Windows is available at: http://127.0.0.1:9105?uri=http://127.0.0.1:65189/_rHWjbz1MZA=/, open the link.
3. Back the the example window, maxmize the window.
4. Check the window height. The number should be 912, not 916.

Questions

1. Why the following code is not in the if (window_manager->IsMaximized()) block

2. Why do you use 9 instead of 8 here?