[Windows] Move `FlutterWindow` mocking to `WindowsProcTable`

[loic-sharma]

Background

The FlutterWindow is one of the fundamental types of the Windows embedder. It listens to the Windows event loop, processes keyboard events, handles IME composition, accessibility, and more.

On top of all of this, it is also used to mock win32 APIs:

• [x] Get DWM composition status (https://github.com/flutter/engine/pull/49397)
• [x] Flush DWM (https://github.com/flutter/engine/pull/49398)
• [ ] Peek and send win32 messages
• [ ] Map virtual key
• [ ] Default window procedure

These are debatable candidates:

• [ ] Clear and present bitmaps

Solution

The WindowsProcTable was introduced to allow mocking win32 APIs. FlutterWindow methods that exist for mocking purposes should be moved to the WindowsProcTable.

[flutter-triage-bot[bot]]

The triaged-desktop label is irrelevant if there is no team-desktop label or fyi-desktop label.