Make javaVersion an enum

Remove the hardcoded checks against "1.9" by making javaVersion
an enum and implementing javaVersion.usesJavaModules().

Test: TestClasspath
Change-Id: I559eeb1f45880bb8177269c6d977ee4dfbadce57
diff --git a/java/aar.go b/java/aar.go
index 6426ac3..2c19487 100644
--- a/java/aar.go
+++ b/java/aar.go
@@ -535,6 +535,10 @@
 	return a.sdkVersion()
 }
 
+func (a *AARImport) javaVersion() string {
+	return ""
+}
+
 var _ AndroidLibraryDependency = (*AARImport)(nil)
 
 func (a *AARImport) ExportPackage() android.Path {