Css fix sizing, add packageId

This patch fixes a few pixels of wiggle on my Android.

It changes the css sizing to border-box which is a way more sane default -- but it may have secondary affects I haven't noticed yet so its worth taking a look at.

I also added a manifest.mobile.json for mobile-chrome-apps usage (that isn't strictly necessary, cca tool will create a default if you don't have one, but you'll need to update packageId manually later).