[Android] Fix crash issue of loading app from manifest with app scheme.

The ManifestReader in Java does not understand the app scheme, so the
url of manifest.json should be convert to android_asset url before
loading the manifest content.

Bug=https://crosswalk-project.org/jira/browse/XWALK-868
Merge pull request #1512 from gaochun/app-scheme-bug

[Android] Fix crash issue of loading app from manifest with app scheme.