Hotspot view model loses state when activity gets recreated

Test instructions:

• Enable the developer option "don't keep activities" (Device Settings → System → Developer options → Apps section → Don't keep activities)
• Open briar
• Go to the share briar offline activity (Briar Settings → Share Briar offline)
• Start sharing (revealing the Wifi configuration for the receiving device)
• Switch to any different app
• Switch back to briar
• Observe that you're back at the intro screen of the offline hotspot feature
• Start sharing again (revealing the Wifi configuration again)
• Make sure that sharing is still working, i.e. connect with a second device and download briar