ContentDialog does not respect BorderThickness

[marcelwgn]

Describe the bug

The ContentDialog does not respect BorderThickness, setting the BorderBrush does work however
Steps to reproduce the bug

Steps to reproduce the behavior:

1. Create ContenDialog with BorderThickness and BorderBrush set to some value

Expected behavior

Both values are respected.
Screenshots

Version Info

WinUI 2.4.2
NuGet package version:

Windows 10 version	Saw the problem?
Insider Build (19628)	Yes
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop
Mobile
Xbox
Surface Hub
IoT

Additional context

[MikeHillberg]

A couple of things going on

• The default ContentDialog control template has a root Grid that sets the BorderThickness to a ThemeResource. It should be TemplateBinding it to the ContentDialog's BorderThickness property, and defaulting that property to the ThemeResource
• ContentDialog's behavior changes if it's in a tree or not. If it's not in a tree, setting the resource takes effect. If it's in a tree there's some resizing logic that causes it to get ignored.