App scheme

[Android] Add app URI scheme support
1 App URI scheme is part of sysapps API, http://app-uri.sysapps.org
2 On Android the local resources is located in assets, which could
be accessed by file:///android_asset/ scheme. So we bridge the app
URI to file URI.
3 Add the HTTP reponse support(200, 400, 403, 404, 501).

BUG=https://crosswalk-project.org/jira/browse/XWALK-174