net: Increase net_mgmt task priority

Originally, the net_mgmt task priority is very low. Based on roaming
implementation, roaming is triggered in net_mgmt task. When running UDP
test while doing roaming, the net_mgmt task won't have much chance to
run and roaming can't be triggered.
Increase it to 3, which is same value of supplicant task.

Signed-off-by: Hui Bai <hui.bai@nxp.com>