[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
Just like in #2226, you can remove the `else` by making the default value be '' before the checks begin.