Bluetooth: Mesh: Ensure seqnum match in app/net

Re-encrypts single-segment application messages when the network seqnum
has changed, to avoid encrypting messages with different seqnums in
network and transport. This operation is only required for unsegmented
messages, as segmented messages don't need to use the same seqnum in
network.

Reinstates the special adv data for friend messages to store the app key
index.

Fixes #19265.

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