[Xamarin.Android.Build.Tasks] delay ToJniName calls in ManifestDocument

[jonathanpeppers]

Reviewing dotnet trace output for a dotnet new maui app:

dotnet trace collect --format speedscope -- .\bin\Release\dotnet\dotnet.exe build -bl --no-restore foo.csproj

8.6% of the time in <GenerateJavaStubs/> is spent in:

93.81ms xamarin.android.build.tasks!Xamarin.Android.Tasks.ManifestDocument.Merge(...)

Drilling down further, a lot of time is spent in:

26.91ms java.interop.tools.javacallablewrappers!Java.Interop.Tools.TypeNameMappings.JavaNativeTypeManager.ToJniName(...)
 4.03ms java.interop.tools.javacallablewrappers!Java.Interop.Tools.TypeNameMappings.JavaNativeTypeManager.ToCompatJniName(...)

It looks like we call this code for every Java.Lang.Object subclass:

var name        = JavaNativeTypeManager.ToJniName (t, cache).Replace ('/', '.');
var compatName  = JavaNativeTypeManager.ToCompatJniName (t, cache).Replace ('/', '.');
if (((string) app.Attribute (attName)) == compatName) {
    app.SetAttributeValue (attName, name);
}

It looks like this code was originally here to solve a problem with Android.App.Application classes that are missing an [Application] attribute:

https://github.com/xamarin/monodroid/commit/2a668b71f821ea0f625626439668c4ee31999526

To improve this:

1. First check if we are a Android.App.Application, apply the above fixup and continue.

2. Find out if we are other types of interesting subclasses, where a generator is needed. continue otherwise.

3. Lastly, compute name & compatName. We are no longer doing this work for every type.

After these changes, I instead get:

72.84ms xamarin.android.build.tasks!Xamarin.Android.Tasks.ManifestDocument.Merge(...)

This saves about ~21ms on incremental builds of dotnet new maui projects.

[jonpryor]

Instead of declaring w/o initializing, I think it would be better to "double declare" them, once within the following if block, and separately within the try block:

if (t.IsSubclassOf ("Android.App.Application", cache)) {
    var (name, compatName) = GetNames (t, cache);
    // …
}
…
try {
    var (name, compatName) = GetNames (t, cache);
    // …
}

[jonathanpeppers]

I put (string name, string compatName) = GetNames (t, cache) so they types are more clear.