Bug 1052442 - [sms] We should never close the activity when viewing the message report. r=julien

Just showing ordinary back button for participants panel and _other_ close button for the report panel + hide _activity_ close button if in activity mode.
The logic is that the report panel is like a dialog on top of the thread, while the participant is another panel located to the right of the thread panel. Normally, we should slide more to the right when opening the participant panel, and then slide left when pressing the "back" button.

The behaviour difference comes from the fact that the report panel is showing information about a specific message while the participant panel is showing information about the thread.