net: lwm2m: lower priority of engine thread

The LwM2M engine thread is used for various periodic triggers.
None of these are in a critical path that requires super sensitive
timing and the current K_PRIO_COOP(7) setting was causing the
Bluetooth RX thread to have to wait too long for certain actions
to complete.

Let's lower the priority to -1 (effectively) to eliminate these
conflicts.

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>