on Android: don't freeze UI while starting challenge, show progress dialog to block input while it's starting;
in shared code: yield "downloading" screen state while _challengeService.ChallengeStore.GetChallenge(...) is executing
Merge branch 'DA-422' into unstable

* DA-422:
  on Android: don't freeze UI while starting challenge, show progress dialog to block input while it's starting; in shared code: yield "downloading" screen state while _challengeService.ChallengeStore.GetChallenge(...) is executing