avoid attr conflict with other library like HoloEverywhere

modify: use android:gravity to replace gravity attr
see http://code.google.com/p/android/issues/detail?id=22576
    https://groups.google.com/forum/#!msg/adt-dev/HeM0aptfddY/R8a18AwE7MQJ
merged "avoid attr conflict with other library like HoloEverywhere" into master

modify: use android:gravity to replace gravity attr
see http://code.google.com/p/android/issues/detail?id=22576

https://groups.google.com/forum/#!msg/adt-dev/HeM0aptfddY/R8a18AwE7MQJ

merged 1687f3e37fedec64f7a3af48670a39f888011b5f