Use the proper Window in multi-window scenarios

[bpulliam]

Description

Since the origin of WinUI controls gallery is single window, many items like StartupWindow which tells the window information are static and don't work in multi window scenario. Instead, we should call xamlroot to find window object as well as app titlebar object when app content has finished loading

Also made a change where dropdown color in Titlebar page changes app titlebar uielement's color instead of titlebar container color. So that it would be painting it below the text instead of over the text.

Motivation and Context

Fix the issue where changes to the Window titlebar were applied to the wrong window.

How Has This Been Tested?

Manual testing

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)