Add missing default true value for 'vector-simplesearch' preference

It was off and only enabled (unconditionally) by the Vector extension
which is being killed off per bug 45051. The preference was added in
r68177, moved to core in r68224.

Change-Id: Id5e96f5ed9dd532b33bc011104388a39f57277dd
(cherry picked from commit 4dd58118ab494960faf9eea54f02c6cd7745c93b)