Bug 16443: Make C4::Members::Statistics plack safe

Bug 16443: Make C4::Members::Statistics plack safe

If the prefs is updated, the fields won't be.
To make sure we already fetch updated values, we should remove the
package variable and define it in the subroutine.

There is not test plan, just make sure the changes are consistent.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 29e402d398f567e2526c89695384bce7f4e4b737)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>