Shut down adb when closing qrenderdoc if it's our adb

* This has some possible problems - e.g. if one copy of qrenderdoc is closed and
  kills the server used by another copy. However it seems impossible to reliably
  manage adb and we need to ensure we don't leave the process lingering around.
* At the moment 95% of users don't use android, so it's most important that they
  have a smooth experience.
* This was the cause of problems around updating versions because adb.exe would
  hang around and then be 'in use' and not able to be overwritten.