code: private boolean hasSession(OmemoDevice userDevice, OmemoDevice contactsDevice) {
          return getOmemoStoreBackend().loadRawSession(userDevice, contactsDevice) != null;
      }
docs: Return true, if the OmemoManager of userDevice has a session with the contactsDevice.
  
  @param userDevice our OmemoDevice.
  @param contactsDevice OmemoDevice of the contact.
  @return true if userDevice has session with contactsDevice.