conversation_module: generate a trust request on 1:1 conv request

If Alice removes Bob and Bob sends a new message, a new request
is received (because Alice doesn't block Bob). However, the
next acceptConversationRequest was not adding correctly Bob into the
contact list, because no trust request were generated (so getContacts()
in this case was returning an invalid list).
This patch ensure that the 1:1 conversation requests are similar to
the trust requests list.

Change-Id: I643d70a016ef549d69ff03a227488b10bfb8d73f