Polished implementation & edge case handling of loading fragment

[Pinselohrkater]

• When no local data is available (prior to 1st successful sync call)

  • [ ] Menu is disabled/invisible. (Cannot open.)
  • [x] Loading spinner is visible and indeterminate/continuous.
  • [x] Distinct text/subtext on fragment:
  • "Preparing the app for first use..."
  • "Bear with us - this shouldn’t take long!"

• When no local data is available , when the sync call did not complete successfully

  • [ ] Menu is disabled/invisible. (Cannot open.)
  • [ ] Loading spinner is replaced with an error icon.
  • [x] Distinct text/subtext on fragment:
  • "Preparing the app for first use..."
  • "Oops! This didn’t work!" + \n\n + Exception message
  • [x] Buttons to Quit (closes app) and Retry (launch update intent again - if it fails it should end up in the very same state)

• When local data is available, while waiting for the sync response

  • [x] Menu is enabled.
  • [x] Loading spinner is visible and indeterminate/continuous.
  • [x] Distinct text/subtext on fragment:
  • "Updating your data..."
  • "You can use the app normally."

• When local data is available, while downloading images

  • [x] Menu is enabled.
  • [x] Loading spinner is visible and determinate/representative of "images to download" vs "images downloaded"
  • [x] Distinct text/subtext on fragment:
  • "Caching content for offline use..."
  • "You can use the app normally."

• When local data is available and sync & image cache have finished

  • [x] Fragment is hidden.