[Xamarin.Android.Build.Tasks] ignore aapt/AndroidManifest.xml

Context: https://stackoverflow.com/questions/41592744/function-of-aapt-androidmanifest-xml-in-aar
Fixes: dotnet#2141

Apparently Android Studio is now shipping a duplicate, pre-formatted
`AndroidManifest.xml` file, inside of AAR files. Its purpose is to be
used with `aapt` invocations, since errors may be thrown related to
`{` or `}` characters.

Since this file may exist in AAR files used by Xamarin.Android, we
should ignore `aapt/AndroidManifest.xml` files the same way we ignore
them inside `manifest` or `bin` directories.

Changes:
- Added a `IgnoredManifestDirectories` list, since we are getting to a
  point where `!= && != && !=` would be a lot of noise.
- Switched the LINQ expression to simple `foreach` loop, which should
  also give a slight performance benefit.

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

@@ -14,6 +14,12 @@ namespace Xamarin.Android.Tasks
 {
 	public class GetImportedLibraries : Task
 	{
+		static readonly string [] IgnoredManifestDirectories = new [] {
+			"bin",
+			"manifest",
+			"aapt",
+		};
+
 		[Required]
 		public string TargetDirectory { get; set; }
 
@@ -37,13 +43,14 @@ public override bool Execute ()
 				return true;
 			}
 			// there could be ./AndroidManifest.xml and bin/AndroidManifest.xml, which will be the same. So, ignore "bin" ones.
-			ManifestDocuments = Directory.GetFiles (TargetDirectory, "AndroidManifest.xml", SearchOption.AllDirectories)
-				.Where (f => {
-					var directory = Path.GetFileName (Path.GetDirectoryName (f));
-					return directory != "bin" && directory != "manifest";
-				})
-				.Select (p => new TaskItem (p))
-				.ToArray ();
+			var manifestDocuments = new List<ITaskItem> ();
+			foreach (var f in Directory.EnumerateFiles (TargetDirectory, "AndroidManifest.xml", SearchOption.AllDirectories)) {
+				var directory = Path.GetFileName (Path.GetDirectoryName (f));
+				if (IgnoredManifestDirectories.Contains (directory))
+					continue;
+				manifestDocuments.Add (new TaskItem (f));
+			}
+			ManifestDocuments = manifestDocuments.ToArray ();
 			NativeLibraries = Directory.GetFiles (TargetDirectory, "*.so", SearchOption.AllDirectories)
 				.Where (p => MonoAndroidHelper.GetNativeLibraryAbi (p) != null)
 				.Select (p => new TaskItem (p)).ToArray ();