[Android][Manifest] Support 'orientation' member

Packaging tool support the 'orientation' member of the manifest:
http://w3c.github.io/manifest/#orientation-member

BUG=https://crosswalk-project.org/jira/browse/XWALK-2022
If orientation is empty string here, then self.orientation = 'unspecified'. Anyway set to unspecified by default here is more readable.