Replace AOSP Calendar with Etar

I have also discussed about this with @thilo on Telegram.

email I sent:

Eelo 0.1 is currently using AOSP Calendar. When I start the Calendar app,
microG pops up and asks for Google login. You should switch to Etar (fork of AOSP Calendar)
or Simple Calendar. Both are available from F-Droid.

Btw, AOSP calendar is also very unmaintained,
since Google is focusing on their Google Calendar app instead of the open-source AOSP one.