Add better exception if user opens second activity

[PureWeen]

Description of Change

• Add better exception when users hit the scenario where they unknowingly launch a new activity but are reusing a window from a previous activity that hasn't been destroyed
• Modify the templates to set LaunchMode to singleTask because they aren't overriding CreateWindow which means they are only compatibile with singleTop launchMode
   

Concern

A few people have worked around this issue by overriding OnPostCreate and ignoring the exception. This PR will break those people but realistically those users should fix the solution correctly opposed to just swallowing the exception.

Issues Fixed

Fixes #18692

[Cybrosys]

• Modify the templates to set LaunchMode to singleTask because they aren't overriding CreateWindow which means they are only compatibile with singleTask launchMode

singleTask -> SingleTop

[mattleibow]

Should LaunchMode = LaunchMode.SingleTop also be set on the blazor template?

[PureWeen]

Should LaunchMode = LaunchMode.SingleTop also be set on the blazor template?

Good question

Let me check with @Eilon and get some thoughts
I'm not sure all the blazor scenarios and if setting that would break anything

[PATRICKdallat]

What release has this been merged into? @PureWeen

[PureWeen]

What release has this been merged into? @PureWeen

8.0.20