fix system page activity grid sorting

The fix for bug 1097094 (commit 49e16e2f) broke the grid sorting because
the .dyn_activity query has a default order_by clause applied, and
@paginate does not clear the existing ordering before it applies its
own.

Bug: 1097094
Change-Id: I18744286cd71530067dcf8475d7e77a1e303cd88