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