[Android] CSS doesn't apply on Crosswalk-based Cordova app.

Because all of the mime-types are replaced by "text/html"
when appending headers for CSP support, even for css file and
image file. These is incorrect. XWalk should use their respective
mime-types for http request. Such as: "image/png" for image file,
"text/css" for css file.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1027