Don't call android.PathForModuleSrc(ctx).String() to get ModuleDir

ctx.ModuleDir is much cheaper than android.PathForModuleSrc(ctx).String().

Test: builds
Change-Id: I81819088d9564b06a0336a59a45f2b110b0bd9d6
diff --git a/java/droiddoc.go b/java/droiddoc.go
index 87588f3..f7d7de6 100644
--- a/java/droiddoc.go
+++ b/java/droiddoc.go
@@ -303,7 +303,7 @@
 	}
 
 	flags = append(flags, android.JoinWithPrefix(aidlIncludes.Strings(), "-I"))
-	flags = append(flags, "-I"+android.PathForModuleSrc(ctx).String())
+	flags = append(flags, "-I"+ctx.ModuleDir())
 	if src := android.ExistentPathForSource(ctx, ctx.ModuleDir(), "src"); src.Valid() {
 		flags = append(flags, "-I"+src.String())
 	}