Bluetooth: Mesh: Fix & clean up Friendship Credential handling

Pass the subnet to some friend_cred_* APIs since it contains all
necessary information for choosing the right keys to generate them
from. Also shorten the API names to avoid awkward line splitting -
these are internal APIs so it's an acceptable compromise. One bug that
this fixes as part of the cleanup is using the right NetKey Index when
clearing Friendship: previously the code was always using the index of
the first subnet, regardless of which subnet the Friendship was based
on.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>