Bluetooth: Mesh: Encrypt friend packets on send

Stores friend queue packets unencrypted, removing any out-of-order
issues caused by seqnum allocation. Also moves as much of the metadata
storage as possible into the packet, allowing us to free up some bytes
of net_buf user data for friend packets.

Fixes #18488

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>