try this

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Resource.Designer.targets

@@ -82,10 +82,14 @@ Copyright (C) 2016 Xamarin. All rights reserved.
     Inputs="@(_AndroidResourceDest);@(LibraryResourceDirectories->'%(StampFile)')"
     Outputs="$(_GenerateResourceCaseMapFile)"
   >
+  <ItemGroup>
+    <!-- Only use the aar files if we have not extracted the data -->
+    <_LocalAarFiles Include="@(AndroidAarLibrary)" Condition=" '@(LibraryResourceDirectories->Count())' == '0' " />
+  </ItemGroup>
   <!-- Generate a ResourceMap file for the project and its resources -->
   <GenerateResourceCaseMap
       AdditionalResourceDirectories="@(LibraryResourceDirectories)"
-      AarLibraries="@(AndroidAarLibrary)"
+      AarLibraries="@(_LocalAarFiles)"
       OutputFile="$(_GenerateResourceCaseMapFile)"
       ProjectDir="$(ProjectDir)"
       ResourceDirectory="$(MonoAndroidResDirIntermediate)"

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

@@ -82,6 +82,7 @@ public override bool RunTask ()
 							continue;
 						if (entry.FullName != resmap)
 							continue;
+						Log.LogDebugMessage ($"Found: {entry.FullName}");
 						using var ms = new MemoryStream();
 						entry.Extract (ms);
 						ms.Position = 0;
@@ -94,6 +95,8 @@ public override bool RunTask ()
 							string [] tok = line.Split (';');
 							AddRename (tok [1].Replace ('/', Path.DirectorySeparatorChar), tok [0].Replace ('/', Path.DirectorySeparatorChar));
 						}
+						// no need to read the rest of the files we found the one we want
+						break;
 					}
 				}
 			}

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/DesignerTests.cs

@@ -147,14 +147,14 @@ public void IncrementalDesignTimeBuild ()
 				Assert.IsTrue (b.Build (proj, parameters: DesignerParameters), $"{b.Target} should have succeeded.");
 
 				// Change a layout, DTB
-				proj.LayoutMain = proj.LayoutMain.Replace ("@string/hello", "hello");
+				proj.LayoutMain = proj.LayoutMain.Replace ("</LinearLayout>", "<Button android:id=\"foo\" android:text=\"hello\" /></LinearLayout>");
 				proj.Touch ("Resources\\layout\\Main.axml");
 				Assert.IsTrue (b.DesignTimeBuild (proj, target: "UpdateGeneratedFiles"), "DTB should have succeeded.");
 
-				var resourcepathscache = Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath, "designtime", "libraryprojectimports.cache");
-				FileAssert.Exists (resourcepathscache);
-				var doc = XDocument.Load (resourcepathscache);
-				Assert.AreEqual (54, doc.Root.Element ("Jars").Elements ("Jar").Count (), "libraryprojectimports.cache did not contain expected jar files");
+				var rTxtFile = Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath, "designtime", "R.txt");
+				FileAssert.Exists (rTxtFile);
+				var doc = File.ReadAllText (rTxtFile);
+				StringAssert.Contains ("foo", doc, "R.tct did not contain expected id of foo");
 			}
 		}
 

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

@@ -1,5 +1,6 @@
 using System;
 using System.CodeDom;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -332,8 +333,8 @@ void ProcessStyleable (XmlReader reader, Dictionary<string, ICollection<R>> reso
 						fields.Add (r);
 					}
 				}
-				if (field.Type != RType.Array)
-					return;
+				//if (field.Type != RType.Array)
+				//	return;
 				arrayMapping.Add (field, fields.ToArray ());
 
 				field.Ids = new int [attribs.Count];