Fix javaOutputDir changed to the JAR, which was breaking unit tests

internal visibility, because they did not get the JAR into classpath

Issue #KT-20212 Fixed

(cherry picked from commit 19e861b)

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/AndroidProject/Lib/src/test/kotlin/internalTest.kt

@@ -0,0 +1,6 @@
+import lib.internalLibFun
+
+fun main(args: Array<String>) {
+    // check that unit tests can access internals of the main variant:
+    internalLibFun()
+}

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/LegacyAndroidAndroidProjectHandler.kt

@@ -61,9 +61,9 @@ internal class LegacyAndroidAndroidProjectHandler(kotlinConfigurationTools: Kotl
         createSyncOutputTask(project, kotlinTask, javaTask, kotlinAfterJavaTask, getVariantName(variantData))
 
         // In lib modules, the androidTest variants get the classes jar in their classpath instead of the Java
-        // destination dir. To use it as a friend path, set the jar as the javaOutputDir (see its usages):
+        // destination dir. Attach the JAR to be consumed as friend path:
         if (variantData is LibraryVariantData) {
-            variantData.dependencyJarOrNull?.let { kotlinTask.javaOutputDir = it }
+            variantData.dependencyJarOrNull?.let { jar -> kotlinTask.attachClassesDir { jar } }
         }
     }