Fix security vulnerability with broadcasts

[caarmen]

https://support.google.com/faqs/answer/9267555

Scenario to reproduce the security vulnerability:

• Launch the app and stay in the main screen
• From another app, schedule (within 10 seconds for example) to launch the following:

  val intent = Intent("ca.rmen.nounours.action.SAVE_ANIMATION")
  intent.putExtra("ca.rmen.nounours.extra.SHARE_INTENT", Intent(Intent.ACTION_VIEW, Uri.parse("https://www.example.com")))
  sendBroadcast(intent)

• Go back to the this app (nounours/bugdroid buddy) before the 10 seconds
• Wait
• Expected behavior: nothing
• Actual behavior: the browser opens in https://www.example.com

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Also: We're required to update the target sdk to at least 29 to publish an update.

Update it to 30, and update the notifications of the IntentService, to use a foreground service during the file export.