profile: add last modified date

The current way to transfer profiles is bugguy with multi-devices.
It was only working before because Desktop client didn't update the
vcard if current one got an avatar...
If two devices exists, they can exchange in loop their profile by
sending to each-other approximately at the same time...
Including the last modified time avoids to override the current
profile with an old one.

jami-project#1558
Change-Id: Iba8fd879c225bd841f915538e3c1678e089cf659