[api-merge] Update "constant" values if they change between API levels.

[jpobst]

Context: https://github.com/xamarin/xamarin-android/issues/9000

In https://github.com/xamarin/xamarin-android/issues/9000, we learned that:

• Constant values can change between Android API levels
• api-merge does not update constant values in the final api.xml if they do change

Update api-merge to update constant values if they change, and update api-VanillaIceCream.xml with the new api-merge logic.

[jpobst]

With this updated api-VanillaIceCream.xml, the only reported constant mismatch is the one reported in #9000, which was caused by an error in our enumification process:

Mismatch: android/app/ApplicationExitInfo.REASON_OTHER 10 != 13 [API-30]

[jonpryor]

Draft commit message:

Context: https://github.com/xamarin/xamarin-android/issues/9000
Context: ac3b405759dfcb3f24a551e8f408ed551b78612f

In Issue #9000, we learned that:

  - Constant values can change between Android API levels
  - `api-merge` does not update constant values in the final `api.xml`
    if they do change

Update `api-merge` to update constant values if they change, and update
`api-VanillaIceCream.xml` with the new `api-merge` logic.

[jonpryor]

@jpobst wrote:

With this updated api-VanillaIceCream.xml, the only reported constant mismatch is the one reported in #9000, which was caused by an error in our enumification process:

Mismatch: android/app/ApplicationExitInfo.REASON_OTHER 10 != 13 [API-30]

Where does this "Mismatch" message come from? I don't see anything in this PR which would emit such a message, and git grep 'Mismatch: ' doesn't find anything either.

[jpobst]

It is a quick local script I wrote that compares map.csv to api.xml.

We can leave the bones of it here for future reference:

class MismatchedEnumValues
{
    static string enum_csv = @"C:\code\xamarin-android\src\Mono.Android\map.csv";
    static string api_xml = @"C:\code\xamarin-android-backport\src\Mono.Android\Profiles\api-VanillaIceCream.xml";

    public static void Do ()
    {
        var consts = ConstantsParser.FromEnumMapCsv (enum_csv).Where (c => c.Action.In (ConstantAction.Enumify)).ToList ();

        var xml = new XmlDocument ();
        xml.Load (api_xml);

        foreach (var c in consts) {
            var package = c.JavaPackage.StartsWith ("I:") ? c.JavaPackage.Substring (2) : c.JavaPackage;
            var elem = xml.SelectSingleNode ($"/api/package[@name='{package.Replace ('/', '.')}']/*[@name='{c.JavaType.Replace ('$', '.')}']/field[@name='{c.JavaName}']");

            if (elem is null) {
                Console.WriteLine ($"Orphan: Could not find {c.JavaPackage}.{c.JavaType}.{c.JavaName} [API-{c.ApiLevel}]");
                continue;
            }

            var value = elem.Attributes? ["value"]?.Value ?? "";

            if (value != c.Value)
                Console.WriteLine ($"Mismatch: {c.JavaPackage}/{c.JavaType}.{c.JavaName} {c.Value} != {value} [API-{c.ApiLevel}]");
        }
    }
}

(Requires a reference to Java.Interop.Tools.Generator project.)