we will have two different manifest files in PROFILE/webapps/keyboard/manifest.webapp and manifest.webapp in applications.zip if we write manifest.webapp here.

I suggest you return manifest object here and write manifest file in webapp-manifest.js, then use `addEntryPointsToManifest` in webapp-zip.js (please reference [webapp-zip.js#L282-L286](https://github.com/mozilla-b2g/gaia/blob/master/build/webapp-zip.js#L282-L286)), get the manifest object and add it into zip by `addToZip`.