[Android] Support 'xwalk_description' field in manifest.

Support 'xwalk_description' field in manifest according to W3C manifest spec.
The legacy field 'description' are still supported, but will print a warning message.
```
if 'description' in self.data_src:
  PrintDeprecationWarning('description')

if 'xwalk_description'  in self.data_src:
  ret_dict['description'] = self.data_src['xwalk_description']
else:
  ret_dict['description'] = self.data_src.get('description', '')
```