rework the change

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Xamarin.Android.DesignTime.targets

@@ -46,8 +46,8 @@ This file is used by all project types, including binding projects.
       <_AndroidLibraryImportsCache Condition=" '$(DesignTimeBuild)' == 'true' And !Exists ('$(_AndroidLibraryImportsCache)') ">$(_AndroidLibraryImportsDesignTimeCache)</_AndroidLibraryImportsCache>
       <_AndroidLibraryProjectImportsCache Condition=" '$(DesignTimeBuild)' == 'true' And !Exists ('$(_AndroidLibraryProjectImportsCache)') ">$(_AndroidLibraryProjectImportsDesignTimeCache)</_AndroidLibraryProjectImportsCache>
       <_AndroidBuildPropertiesCache Condition=" '$(DesignTimeBuild)' == 'true' ">$(_AndroidDesignTimeBuildPropertiesCache)</_AndroidBuildPropertiesCache>
-      <_GeneratorStampFile Condition=" '$(DesignTimeBuild)' == 'true' ">$(_AndroidIntermediateDesignTimeStampDirectory)generator.stamp</_GeneratorStampFile>
-      <_AndroidResgenFlagFile Condition=" '$(DesignTimeBuild)' == 'true' ">$(_AndroidIntermediateDesignTimeBuildDirectory)R.cs.flag</_AndroidResgenFlagFile>
+      <_GeneratorStampFile Condition=" '$(DesignTimeBuild)' == 'true' And !Exists('$(_GeneratorStampFile)') ">$(_AndroidIntermediateDesignTimeStampDirectory)generator.stamp</_GeneratorStampFile>
+      <_AndroidResgenFlagFile Condition=" '$(DesignTimeBuild)' == 'true' And !Exists('$(_AndroidResgenFlagFile)') ">$(_AndroidIntermediateDesignTimeBuildDirectory)R.cs.flag</_AndroidResgenFlagFile>
     </PropertyGroup>
     <MakeDir
         Condition=" '$(_AndroidIsBindingProject)' != 'true' And !Exists ('$(_AndroidIntermediateDesignTimeBuildDirectory)') "

tests/MSBuildDeviceIntegration/Tests/InstantRunTest.cs

@@ -103,7 +103,7 @@ public void TargetsSkipped ([Values(false, true)] bool useManagedResourceGenerat
 				// CoreCompile should be built.
 				Assert.IsTrue (b.Output.IsTargetSkipped ("CoreCompile"), "5 CoreCompile was not skipped");
 			} else {
-				Assert.IsTrue (!b.Output.IsTargetSkipped ("_UpdateAndroidResgen"), "5 second _UpdateAndroidResgen was skipped");
+				Assert.IsTrue (b.Output.IsTargetSkipped ("_UpdateAndroidResgen"), "5 second _UpdateAndroidResgen was not skipped");
 				// CoreCompile should not be built.
 				Assert.IsTrue (b.Output.IsTargetSkipped ("CoreCompile"), "5 CoreCompile was not skipped");
 			}