mgmt: mcumgr: Make signed thread priorities enabled by default

By default, thread priorities in mcumgr task stat responses are
unsigned, whilst in zephyr, thread priorities are signed. This means
that clients get obscenely large numbers for priorities that make no
sense. The fork of mcumgr has been in zephyr long enough now that
this should be changed to use signed thread priorities by default
instead of sticking with the old mcumgr default.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>