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

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