[Android] "version" field in manifest should not be mandatory and should...

... be renamed to "xwalk_version"

According W3C manifest spec, 'version' field of manifest should be:
1) renamed to "xwalk_version"
2) optional
3) the "version" field should be handled if present, but issue a deprecation warning

BUG=https://crosswalk-project.org/jira/browse/XWALK-2053
```
if 'version' in self.data_src:
  PrintDeprecationWarning('version')
  
if 'xwalk_version' in self.data_src:
  ret_dict['xwalk_version'] = self.data_src['xwalk_version']
else:
  ret_dict['version'] = self.data_src.get('version', '')