Bluetooth: Mesh: Fix err in mod_app_list packing

Fixes erroneous packing/unpacking of model app list messages
in the configuration client and server. According to the mesh 1.1
protcol spec (4.3.1.1) two app indexes shall be packed in a 3 octet
interleaved format. The current implementation packs them in 4 octets.

This commit also provide a helper function for unpacking key indexes
as public API to facilitate future config model callback API.

Signed-off-by: Anders Storrø <anders.storro@nordicsemi.no>