[Android] Add tag to generate AAR library

use command "ninja -C out/Release xwalk_core_library_aar" can generate AAR library,
the AAR library test successfully by gradle. 

the construct as follow
xwalk_core_library.aar
├── jni
│   └── x86
│    |   └──so
     └── armeabi-v7a
│           ├── so
├── res
├── classes.jar
├── R.txt
├── AndroidManifest.xml

BUG=XWALK-1930

missing two blank lines here