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

@@ -4299,37 +4299,37 @@ if (enable_java_templates) {
       not_needed(invoker, [ "strip_drawables" ])
     }
 
-    _java_library_vars = [
-      "enable_bytecode_checks",
-      "enable_jetify",
-      "jar_excluded_patterns",
-      "jar_included_patterns",
-      "missing_classes_allowlist",
-      "requires_android",
-      "testonly",
-    ]
+    # Create android_java_prebuilt target for classes.jar.
+    if (_scanned_files.has_classes_jar) {
+      _java_library_vars = [
+        "enable_bytecode_checks",
+        "enable_jetify",
+        "jar_excluded_patterns",
+        "jar_included_patterns",
+        "missing_classes_allowlist",
+        "requires_android",
+        "testonly",
+      ]
 
-    # Create android_java_prebuilt target for extra jars within jars/.
-    _subjar_targets = []
-    foreach(_tuple, _scanned_files.subjar_tuples) {
-      _current_target = "${target_name}__subjar_${_tuple[0]}"
-      _subjar_targets += [ ":$_current_target" ]
-      java_prebuilt(_current_target) {
-        forward_variables_from(invoker, _java_library_vars)
-        deps = [ ":$_unpack_target_name" ]
-        if (!defined(requires_android)) {
-          requires_android = true
+      # Create android_java_prebuilt target for extra jars within jars/.
+      _subjar_targets = []
+      foreach(_tuple, _scanned_files.subjar_tuples) {
+        _current_target = "${target_name}__subjar_${_tuple[0]}"
+        _subjar_targets += [ ":$_current_target" ]
+        java_prebuilt(_current_target) {
+          forward_variables_from(invoker, _java_library_vars)
+          deps = [ ":$_unpack_target_name" ]
+          if (!defined(requires_android)) {
+            requires_android = true
+          }
+          supports_android = true
+          jar_path = "$_output_path/${_tuple[1]}"
+          _base_output_name = get_path_info(jar_path, "name")
+          output_name = "${invoker.target_name}-$_base_output_name"
+          public_target_label = invoker.target_name
         }
-        supports_android = true
-        jar_path = "$_output_path/${_tuple[1]}"
-        _base_output_name = get_path_info(jar_path, "name")
-        output_name = "${invoker.target_name}-$_base_output_name"
-        public_target_label = invoker.target_name
       }
-    }
 
-    # 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)