Dialer: Bring back merge calls

* Commit acd66498 removed speakeasy references
* Showing the merge button should ofc not depend on all calls being
  null, so the logic is now "always show merge button if the capability
  is there", as it should be

Test:
- Call someone
- Press "Add call"
- Call someone else
- Merge button is there and makes it a conference

Fixes: https://gitlab.com/LineageOS/issues/android/-/issues/6332
Change-Id: Ifd8f8bc0f82b0b66fa28272e1c18a2467eed8e72