[Android] Use loadAppFromManifest if a manifest.json is used for a web app.
(cherry picked from commit e345cca9cdb4363557feae8a6929727592b37f92)

BUG=https://crosswalk-project.org/jira/browse/XWALK-315
Merge pull request #1215 from kurli/v2-beta-xwalk-build-tool-fix

[Android] Use loadAppFromManifest if a manifest.json is used for a web app