minor tweaks on prothesis window-manager.js
prevents b2g stylsheet inject side-effect on running an app during b2g startup

- move b2g stylesheet injection code into its own file (inject-b2g-agentsheet.js)
- observe "content-document-global-created" notification before ContentStart to
  inject into "iframe#homescreen" created inside shell.xul
  (prevents layouting bug on running app during b2g startup)