[One .NET] prefer /manifest/@android:versionCode

[jonathanpeppers]

Fixes: #6384
Fixes: dotnet/sdk#21614

In .NET 6, you can't currently put versionCode in
AndroidManifest.xml, as it is always overwritten by a default:

<ApplicationVersion Condition=" '$(ApplicationVersion)' == '' ">1</ApplicationVersion>

We should actually check if the value is present in
AndroidManifest.xml and only set if blank.

The other new MSBuild properties don't appear to suffer from this
problem:

• $(ApplicationId)
• $(ApplicationTitle)
• $(ApplicationDisplayVersion)

This is because we don't default these properties to a value when they
are blank.

I added a new test for this scenario.

[jpobst]

It kinda feels like if you put a version in both <ApplicationVersion> and AndroidManifest.xml, the <ApplicationVersion> version should win, as it is much more flexible, especially for CI scenarios.

Is there a way to set the precedence to:

• User set <ApplicationVersion>
• AndroidManifest.xml
• Default <ApplicationVersion>

[jonathanpeppers]

The spec says the native platform files always win:

https://github.com/xamarin/xamarin-android/blob/main/Documentation/guides/OneDotNetSingleProject.md

If you want to use the MSBuild properties, you can just leave the values blank in Info.plist or AndroidManifest.xml (or whatever the WinUI version is). You can still set MSBuild properties in .csproj files and override their values from CI.

I updated the test case some -- it needs to set all the properties to weird values so we see who wins.