[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 you decide to warn different messages for those two cases, you should put `https://crosswalk-project.org/jira/browse/XWALK-2138` as the first condition to avoid showing the warning for both messages.