Fix kotlin -classpath argument

kotlinc expects -classpath, not --classpath.

Also add a test that uses only .kt files to exercise the code when
there are no java sources.

Test: java_test.go
Change-Id: Ifa5a007b460b40ea2188d0907570fbdca6c48da7

java/builder.go

@@ -203,7 +203,7 @@ func TransformKotlinToClasses(ctx android.ModuleContext, outputFile android.Writ
 		Output:      outputFile,
 		Inputs:      inputs,
 		Args: map[string]string{
-			"classpath":    flags.kotlincClasspath.FormJavaClassPath("--classpath"),
+			"classpath":    flags.kotlincClasspath.FormJavaClassPath("-classpath"),
 			"kotlincFlags": flags.kotlincFlags,
 			"outDir":       classDir.String(),
 			"javaVersion":  flags.javaVersion,