Remove IBrowserHostServices/INativeProgressPage and cleanup AppSecurityManager

[h3xds1nz]

Description

Another batch of XBAP-related code removal; continues #9865 and previous PRs.

• Removes IBrowserHostServices fully as it now had 1 unused enum left and that was it.
• Removes INativeProgressPage and it was used in XappLauncherApp which is no longer present.
• Removes IsAllowedScriptScheme function in WebBrowserEvent.
  • This is not part of the interface, and the calling code was removed when transitioning to .NET Core.
• Cleans up the code in AppSecurityManager and removes the obsolete method overloads.
• Sealed the SiteOfOriginContainer class
  • Cleans up comments/regions (suddenly the file is readable)
  • Put BooleanSwitch behind property, make it readonly
• Regarding AppDomain.CurrentDomain.BaseDirectory change:
  • Since there are no AppDomains on .NET Core, CurrentDomain is a singleton and the BaseDirectory property just redirects to AppContext.BaseDirectory. As part of ongoing effort, I've replaced it here.

Customer Impact

Decreased size of assemblies, tiny bit of perf, cleaner codebase for developers.

Regression

No.

Testing

Local build, app run.

Risk

Low, the code is a dead code or it doesn't do anything.

Microsoft Reviewers: Open in CodeFlow