Remove bus image from stop icon, in favor of a clean circle

* As discussed in https://github.com/OneBusAway/onebusaway-android/issues/155#issuecomment-65545104
* Allows us to use bus image for future icons that denote real-time bus position on the map
* This drawable circle could also be dynamically recolored (at build or runtime) as part of re-branding the app, without needing to regenerate PNG assets.