Use ContactListViewModel in ContactChooserFragment and IntroductionActivity

Closes #1905 (closed)

Test instructions:

1. Check that contact list used displays correctly
   • Go into a conversation
   • menu → make introduction
   • check that list does not contain the contact you're introducing
2. Check that contact list used updates properly
   • On some device (A) repeat steps from 1.
   • While the contact selection list is open: use a different device (B) and sent a message to (A). Make sure the number of unread messages displayed on (A) updates and also the contact moves up to the top as it now is the contact with the last recent message
3. Check that selecting a contact works
   • Repeat steps from 1.
   • select a contact and check that the next screen show the expected contacts