Bluetooth: host: Don't send ATT_MULTIPLE_HANDLE_VALUE_NTF with one attr

Sending ATT_MULTIPLE_HANDLE_VALUE_NTF containing only one handle doesn't
seem forbidden by the spec, but peers might find it unexpected.

This change morphs a ATT_MULTIPLE_HANDLE_VALUE_NTF into a
ATT_HANDLE_VALUE_NTF right before sending it, if it detects this
condition.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>