Eliminate last_ticks from TIMER_LIST_ENT.

It's unnecessary, potentially calculated incorrectly, and the insertion
logic that depended on it was fragile. I've rewritten the list
insertion routine so it's easier to follow. This should also fix
http://b/16259295.

Change-Id: Ib7184c94c495c03d3acbe105955b1bb712dbbfaa
(cherry picked from commit 7af0bd9c)