Use R.txt from AAR to generate R.java for it when building APK.

build/android/gyp/aar.py

Index: build/android/gyp/aar.py
diff --git a/build/android/gyp/aar.py b/build/android/gyp/aar.py
index 566cab37d98d7c3f02a9c3e1b1d5b742f01222ed..d58c5a80a2f16b0fead52fa30b1bfec83943c2f2 100755
--- a/build/android/gyp/aar.py
+++ b/build/android/gyp/aar.py
@@ -77,6 +77,7 @@ def main():
     data['has_classes_jar'] = False
     data['has_proguard_flags'] = False
     data['has_native_libraries'] = False
+    data['has_r_text_file'] = False
     with zipfile.ZipFile(aar_file) as z:
       data['is_manifest_empty'] = (
           _IsManifestEmpty(z.read('AndroidManifest.xml')))
@@ -101,6 +102,8 @@ def main():
           data['has_classes_jar'] = True
         elif name == 'proguard.txt':
           data['has_proguard_flags'] = True
+        elif name == 'R.txt':
+          data['has_r_text_file'] = True
 
     print gn_helpers.ToGNString(data)