ui: fix notifications

When receiving many text messages, as many notifications were created.
Now, there is one notification for every conversation with new
messages.

If you received messages from several senders, the ringID was always
the same. This commit fixes this behavior.

Change-Id: I8c54a7cca1ad395f3b3f9f5fba32c832cfd893bf
Tuleap: #1414