Allow android_aar_prebuilt to support resource only aars

Using android_aar_prebuilt for an aar that does not contain a
classes.jar will fail gn with unused variables. Only define
those variables if needed.

Change-Id: I6d3776271e7bc817329af7f8976fd5719f581bac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2236036Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Auto-Submit: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776116}

build/config/android/rules.gni

@@ -4299,6 +4299,8 @@ if (enable_java_templates) {
       not_needed(invoker, [ "strip_drawables" ])
     }
 
+    # Create android_java_prebuilt target for classes.jar.
+    if (_scanned_files.has_classes_jar) {
       _java_library_vars = [
         "enable_bytecode_checks",
         "enable_jetify",
@@ -4328,8 +4330,6 @@ if (enable_java_templates) {
         }
       }
 
-    # Create android_java_prebuilt target for classes.jar.
-    if (_scanned_files.has_classes_jar) {
       _jar_target_name = "${target_name}__classes"
       java_prebuilt(_jar_target_name) {
         forward_variables_from(invoker, _java_library_vars)