notifications: fix invitations

When the LIBJAMI_FLAG_NO_AUTOSYNC flag is used, it is still
necessary to call startSync in order to receive invitations.
This patch applies the LIBJAMI_FLAG_NO_AUTOSYNC flag to startSync
callbacks.

jami-client-ios#280

Change-Id: Ieb8a1fb8542c4caf975e13d5a3210bdbc1099a55