Switch to a borderless icon for Chrome apps

Turns out Chrome doesn’t stick to the 128x128 icon when provided, so
need to make them all consistent. And since iOS and Android icons get
automatically rounded corners it’s best to use the borderless version.

Version w/ border still used for the favicon.
Stop manually copying manifest.mobile.json

Now kept next to manifest.json so the version number can be incremented
atomically (hopefully).