Bluetooth: Host: Implement dynamic apperance

Introduces new kconfig option CONFIG_BT_DEVICE_APPEARANCE_DYNAMIC.

New API `bt_set_apperance` allows dynamic setting of apperance. The
dynamic setting is saved in the settings subsys. `bt_set_apperance` is
analogous to `bt_set_name`.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>