Better Reset Handling

* Reset the Emulator (and adb) on network error.
  This ensures a “clean slate” for the reconnection.
* Forget about which assets have been loaded to the
  Companion when network connection is reset. This
  solves the problem where people disconnect from
  an emulator and then connect to a phone and don’t
  see their assets (because we think they are loaded,
  not understanding that the actual device has been
  replaced).

Change-Id: I46c06ad4f32c1fc92f3b098f403b86d880b12661