[Xamarin.Android.Build.Tasks] Scan for JCWs for each ABI in parallel (#9215)

In order to ensure that all of our per-arch assemblies contain the
same Java Callable Wrapper types, we scan them all.

We can parallelize this operation per-ABI to save time.

*Note*: It is ok to parallelize Cecil here, as each "set" contains
completely different assemblies coming from completely different
assembly resolvers.

Fresh `dotnet build` of Android template (defaults to 4 architectures),
total time `GenerateJavaSourcesAndMaybeClassifyMarshalMethods` takes:

| Commit    | Time (ms) |
| :-------- | -------: |
| 48239a5  | 11835 ms |
| PR        |  4548 ms |

Author: jpobst

src/Xamarin.Android.Build.Tasks/Tasks/GenerateJavaStubs.cs

@@ -16,6 +16,8 @@
 using Xamarin.Android.Tools;
 using Microsoft.Android.Build.Tasks;
 using Java.Interop.Tools.JavaCallableWrappers.Adapters;
+using System.Threading.Tasks;
+using System.Collections.Concurrent;
 
 namespace Xamarin.Android.Tasks
 {
@@ -183,26 +185,38 @@ void Run (bool useMarshalMethods)
 			}
 
 			// Now that "never" never happened, we can proceed knowing that at least the assembly sets are the same for each architecture
-			var nativeCodeGenStates = new Dictionary<AndroidTargetArch, NativeCodeGenState> ();
-			bool generateJavaCode = true;
+			var nativeCodeGenStates = new ConcurrentDictionary<AndroidTargetArch, NativeCodeGenState> ();
 			NativeCodeGenState? templateCodeGenState = null;
 
-			foreach (var kvp in allAssembliesPerArch) {
+			var firstArch = allAssembliesPerArch.First ().Key;
+			var generateSucceeded = true;
+
+			// Generate Java sources in parallel
+			Parallel.ForEach (allAssembliesPerArch, (kvp) => {
 				AndroidTargetArch arch = kvp.Key;
 				Dictionary<string, ITaskItem> archAssemblies = kvp.Value;
+
+				// We only need to generate Java code for one ABI, as the Java code is ABI-agnostic
+				// Pick the "first" one as the one to generate Java code for
+				var generateJavaCode = arch == firstArch;
+
 				(bool success, NativeCodeGenState? state) = GenerateJavaSourcesAndMaybeClassifyMarshalMethods (arch, archAssemblies, MaybeGetArchAssemblies (userAssembliesPerArch, arch), useMarshalMethods, generateJavaCode);
 
 				if (!success) {
-					return;
+					generateSucceeded = false;
 				}
 
+				// If this is the first architecture, we need to store the state for later use
 				if (generateJavaCode) {
 					templateCodeGenState = state;
-					generateJavaCode = false;
 				}
 
-				nativeCodeGenStates.Add (arch, state);
-			}
+				nativeCodeGenStates.TryAdd (arch, state);
+			});
+
+			// If we hit an error generating the Java code, we should bail out now
+			if (!generateSucceeded)
+				return;
 
 			if (templateCodeGenState == null) {
 				throw new InvalidOperationException ($"Internal error: no native code generator state defined");

src/Xamarin.Android.Build.Tasks/Tasks/GeneratePackageManagerJava.cs

@@ -13,6 +13,7 @@
 using Java.Interop.Tools.TypeNameMappings;
 using Xamarin.Android.Tools;
 using Microsoft.Android.Build.Tasks;
+using System.Collections.Concurrent;
 
 namespace Xamarin.Android.Tasks
 {
@@ -313,9 +314,9 @@ void AddEnvironment ()
 				}
 			}
 
-			Dictionary<AndroidTargetArch, NativeCodeGenState>? nativeCodeGenStates = null;
+			ConcurrentDictionary<AndroidTargetArch, NativeCodeGenState>? nativeCodeGenStates = null;
 			if (enableMarshalMethods) {
-				nativeCodeGenStates = BuildEngine4.GetRegisteredTaskObjectAssemblyLocal<Dictionary<AndroidTargetArch, NativeCodeGenState>> (
+				nativeCodeGenStates = BuildEngine4.GetRegisteredTaskObjectAssemblyLocal<ConcurrentDictionary<AndroidTargetArch, NativeCodeGenState>> (
 					ProjectSpecificTaskObjectKey (GenerateJavaStubs.NativeCodeGenStateRegisterTaskKey),
 					RegisteredTaskObjectLifetime.Build
 				);

src/Xamarin.Android.Build.Tasks/Utilities/JCWGenerator.cs

@@ -15,6 +15,7 @@
 using Microsoft.Build.Utilities;
 using Mono.Cecil;
 using Xamarin.Android.Tools;
+using System.Collections.Concurrent;
 
 namespace Xamarin.Android.Tasks;
 
@@ -220,7 +221,7 @@ static string GetMonoInitSource (string androidSdkPlatform)
 		return builder.ToString ();
 	}
 
-	public static void EnsureAllArchitecturesAreIdentical (TaskLoggingHelper logger, Dictionary<AndroidTargetArch, NativeCodeGenState> javaStubStates)
+	public static void EnsureAllArchitecturesAreIdentical (TaskLoggingHelper logger, ConcurrentDictionary<AndroidTargetArch, NativeCodeGenState> javaStubStates)
 	{
 		if (javaStubStates.Count <= 1) {
 			return;