webadmin: resolve findbugs warning in HumanReadableTimeRenderer.

Resolve warning of type
BX_BOXING_IMMEDIATELY_UNBOXED_TO_PERFORM_COERCION

Change-Id: Ia14f49fd458f3384ba4829d2e4fc4301ad6de4ee
Signed-off-by: Alissa Bonas <abonas@redhat.com>