Various HMD Tools improvements

* if we exit the app in full screen, then when we run the next time we get a giant white screen. This is not really related to HDM mode, but Ryan found it while testing new HDM mode. So we leave fullscreen mode when exiting the app.

* also don't stay in HDM mode when exiting the app... this is a good idea because we don't want to leave the app running on the oculus because it would be hard to find on the next startup... so always exit HDM mode before quitting the app.

* also add some guards against running "enter" or "leave" HDM mode multiple times in a row.

* change to a single button that switches modes

* better detection of add/removing screens like plugging in/out the oculus or switching between mirrored or virtual displays. Now we detect when the oculus screen has come or gone and we reset the SDK and "best HMD" screen appropriately.