[Android] Support loading manifest following w3c spec

Spec: http://w3c.github.io/manifest/

This commit is only for the loading API of XWalkView to
work with w3c manifest spec.
For packaging tool's support, it needs additional work.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1920
This is not needed... we dont differentiate between w3c and non-w3c manifests. We should ALWAYS warn if not compliant.