[Android] Rename version to xwalk_version in manifest and make it optional

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