Fixes various issues in WPF Gallery in HC mode

[dipeshmsft]

Description

This PR fixes a few issues in the HighContrast mode for WPF Gallery. Here is the list of issues that are fixed by this PR :

1. When launched in HC mode directly, the gallery applications main window does not have any corner radius and when we switch to normal mode, we still don't see any corner radius.
   • Fixed by using a defined corner radius for the window rather than depending on the default value.
2. When in HC mode, the border around the main window, does not change color when it goes from activated to deactivated or vice versa.
   • Done by handling activated and inactivated events and updating the visuals accordingly.
3. When in HC mode, there was a padding around the main window in maximized state.
4. Due to a recent PR Fix close button misalignment in WindowChrome #698 , when launched in HC mode, the application showed erratic flickering behavior when resizing.
   • This is fixed by switching the NonClientEdges for the window chrome in HC mode.

Customer Impact

WPF Gallery will function properly in HighContrast mode.

Testing

Local testing

Microsoft Reviewers: Open in CodeFlow