endSessionDialog: Factor out _updateDescription from _updateContent

endSessionDialog: Factor out _updateDescription from _updateContent

_startTimer adds a tweener to implement the description countdown, which
updates the entire content on each iteration, including the icon. This
causes a significant impact on performance, especially when accessibility
is enabled, as it causes a flood of AtkObject:state-change:showing events.
As the countdown only affects the description, factor out _updateDescription
and use it in _startTimer, and only do a full update of all contents when
necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=674210