Gradle: set output directory to tmp/kotlin-classes to exclude it in IDE.

This is a temporary workaround: should be removed when Android Studio will get excluded output directories from Gradle Model (see AndroidContentRoot.java:35)
 #KT-4507 Fixed

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

@@ -204,7 +204,7 @@ open class KotlinAndroidPlugin(val scriptHandler: ScriptHandler): Plugin<Project
 
 
                 // store kotlin classes in separate directory. They will serve as class-path to java compiler
-                val kotlinOutputDir = File(project.getBuildDir(), "kotlin-classes/${variantName}")
+                val kotlinOutputDir = File(project.getBuildDir(), "tmp/kotlin-classes/${variantName}")
                 kotlinTask.kotlinDestinationDir = kotlinOutputDir;
                 kotlinTask.setDestinationDir(javaTask.getDestinationDir())
                 kotlinTask.setDescription("Compiles the ${variantName} kotlin.")