[Xamarin.Android.Build.Tasks] remove checks for $(UsingAndroidNETSdk)

[jonathanpeppers]

In .NET 6+, $(UsingAndroidNETSdk) is always true as the general identifier that this project is .NET and not Xamarin.Android. We used this flag as a way to share code between .NET Android and Xamarin.Android.

• Remove checks for $(UsingAndroidNETSdk)

• No longer need to pass in this value to MSBuilds tasks or other C# classes

• Checks if true leave in place, and remove the condition

• Checks if false can just be completely removed now

In a future PR, we may also be able to remove $(_AndroidIsBindingProject), which is slightly related. We removed the "binding project" as a concept in .NET 6+.

I left the $(UsingAndroidNETSdk) property in place, even though it is not used in these targets. Other general MSBuild targets may continue to use it in perpetuity.

[jonathanpeppers]

Ignore the MAUI lane for now, I think their build is broken:

C:\a\_work\3\s\maui\src\Core\src\Platform\Tizen\StackNavigationManager.cs(73,25): error CA1859: Change type of variable 'newPageStack' from 'System.Collections.Generic.IReadOnlyList<Microsoft.Maui.IView>' to 'System.Collections.Generic.List<Microsoft.Maui.IView>' for improved performance (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1859) [C:\a\_work\3\s\maui\src\Core\src\Core.csproj::TargetFramework=net9.0-tizen]
C:\a\_work\3\s\maui\src\Core\src\Platform\Windows\StackNavigationManager.cs(65,25): error CA1859: Change type of variable 'newPageStack' from 'System.Collections.Generic.IReadOnlyList<Microsoft.Maui.IView>' to 'System.Collections.Generic.List<Microsoft.Maui.IView>' for improved performance (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1859) [C:\a\_work\3\s\maui\src\Core\src\Core.csproj::TargetFramework=net9.0-windows10.0.20348]
C:\a\_work\3\s\maui\src\Core\src\Platform\Windows\StackNavigationManager.cs(65,25): error CA1859: Change type of variable 'newPageStack' from 'System.Collections.Generic.IReadOnlyList<Microsoft.Maui.IView>' to 'System.Collections.Generic.List<Microsoft.Maui.IView>' for improved performance (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1859) [C:\a\_work\3\s\maui\src\Core\src\Core.csproj::TargetFramework=net9.0-windows10.0.19041]