Fix AccountInfo.has_avatar_info() that wrongly always returns true

AccountInfo.has_avatar_info() checks whether the AccountInfo contains
an avatar field. This is currently always true. Set this field on
server side only if an avatar provider is installed.

Change-Id: Ib73bd925d70d5049688ae9a57a058645b2cbda41
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>