Embedders can now notify shell during startup about the various displays and their corresponding settings.
Adds a notion of Display update type which can later include chages to displays during runtime such as addition / removal / reconfiguration of displays.

We also remove the responsibility of providing the refresh rate from `vsync_waiter` to `DisplayManager`.
Rewires existing platform implementations of the said API to use `Shell::OnDisplayUpdate` to notify the display manager of the startup configuration.

DisplayManager is also thread-safe to account for rasterizer and UI thread accesses.