Replace droiddoc.custom_template_dir

With an explicit module instead of a full path.

Test: m core-docs
Change-Id: I0eb71e496f4de402016d887361702e7a4ff2c64e
diff --git a/JavaLibrary.bp b/JavaLibrary.bp
index 75d500b..6123e50 100644
--- a/JavaLibrary.bp
+++ b/JavaLibrary.bp
@@ -286,7 +286,7 @@
     exclude_srcs: [
         ":annotated_ojluni_files",
     ],
-    custom_template_dir: "external/doclava/res/assets/templates-sdk",
+    custom_template: "droiddoc-templates-sdk",
     hdf: [
         "android.whichdoc offline",
     ],