Activities try to join not shared session

[alsroot]

Test case:

* A and B have a jobject that was created with the same activity_id (e.g., after joining to A from B and saving jobjects on A and B);
* from newly started A and B, A resumes his jobject;
* it becomes shared;
* A stops activity;
* B resume his jobject;
* activity tries to join (to absent A's session) and can't (scale is private)

The problem is being triggered by calling SetCurrentActivity for org.laptop.Telepathy.BuddyInfo in shell sources. Commenting out all these calls, "fixes" the problem.

Installed Telepathy:

* telepathy-glib-0.14.10
* telepathy-gabble-0.13.6
* telepathy-mission-control-5.8.1

[alsroot]

Seems to be the fix:
 http://git.sugarlabs.org/desktop/sugar-toolkit/commit/589317515cc0cf0ddf9f83014e7e8144d9a89512

[alsroot]

The SetCurrentActivity issue is different, #3425.