convmodule: avoid incorrect states

In some cases the file "contacts" was badly synced. Also, if for any
reason "contacts" is not correctly formed, we may be able to fix it.

This patch handles two cases of failures:

1. If the contact details doesn't contain any conversationId BUT
a 1:1 conversation is found with this contact, we update the
conversationId in the details.

2. If, for some reason, multiple 1:1 conversations are detected
with the same contact, we only keep the one in the details, as
it should be the correct one.

This should fix the fact that for some conversation, calls are not
written in the history (cause getOneToOneConversation() returns
nothing).

Change-Id: I5dd9fc51999540d8a4230f8fdce828a461da752a