Remove `IsRunningOnDesktop` for the Android designer

[jonathanpeppers]

Android framework version

net10.0-android (Preview)

Affected platform version

.NET 10

Description

Context: https://developercommunity.visualstudio.com/t/XamarinAndroid-Designer---Replacement/10728132

The Android designer has been removed from Visual Studio, but we still have some code in the product supporting it.

We should be able to remove:

• JNIEnvInit.IsRunningOnDesktop in C#
• JnienvInitializeArgs.isRunningOnDesktop in C++
• The $__XA_PACKAGE_NAMING_POLICY__ environment variable, as it's only used within a if (IsRunningOnDesktop) block
• The $(AndroidPackageNamingPolicy) MSBuild property should remain as-is, as it is still used within the codebase.

Any if-statements can be inlined / deleted as if IsRunningOnDesktop is always false.

After removing code, update any documentation, error messages, string resources, as needed to reflect the changes.