Use ExistentPathForSource

Converting java to use Path objects incorrectly used PathForModuleSrc
for a path that might not exist, use ExistentPathForSource instead.

Test: java_test.go
Change-Id: I50e4e315a10ab08c4e72065094ae29844874f5e4
diff --git a/java/java.go b/java/java.go
index 3cc552f..adb3d7b 100644
--- a/java/java.go
+++ b/java/java.go
@@ -203,7 +203,9 @@
 	flags = append(flags, android.JoinWithPrefix(j.exportAidlIncludeDirs.Strings(), "-I"))
 	flags = append(flags, android.JoinWithPrefix(localAidlIncludes.Strings(), "-I"))
 	flags = append(flags, "-I"+android.PathForModuleSrc(ctx).String())
-	flags = append(flags, "-I"+android.PathForModuleSrc(ctx, "src").String())
+	if src := android.ExistentPathForSource(ctx, "", "src"); src.Valid() {
+		flags = append(flags, "-I"+src.String())
+	}
 
 	return flags
 }