Track reading list size
Avoid costly count(*) queries by keeping track of the size of the
list. This is a temporary solution until T177462 is sorted out.
Bug: T187226
Change-Id: I74a56f8600ec2172805e0cf5ed9431b7a730d783
(cherry picked from commit ff5c38403e6e62b175ba829ba14a3f1622c15a56)