Your example works if you replace
local feedback_html = sys.load_resource("file:///android_asset/www/index.html")
webview.open_raw(webview_id, feedback_html)
with
webview.open(webview_id, "file:///android_asset/www/index.html")
Everything you put into asset folder in apk (in Defold we use bundle_resources for that) is available by file:///android_asset/
For example if you try
webview.open(webview_id, "file:///android_asset/game.projectc")
You will see your game.project file =)