bluetooth: host: Persist Service Changed data

Add support for persisted Service Changed data, to fix the case of a
paired device not reconnecting before a reboot and thus not receiving
SC indication. It also enables support for GATT database being changed
during a firmware update.

Move Service Changed data outside of the CCC struct and make it
persistent by adding support for a bt/sc/... setting.

Signed-off-by: François Delawarde <fnde@oticon.com>