Bluetooth: host: Move pending keys pending flags out of key storage

Move the pending ID keys add and delete flag out of keys storage area.
These flags are runtime flags and should not be stored in persistent
storage.
Due to struct alignment storage start has to be aligned so that
variables added before storage start does not affect the storage bytes
by introducing padding in the storage area

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>