Bluetooth: host: Optionally disable GATT sec re-establishment

Some centrals deal poorly with receiving a security request immediately
after reconnection whenever reconnecting with characteristics that are
notifiable or indicatable and requiring security. In particular,
Android 9 and earlier devices may lose bond information when this
happens, some Microsoft Surface devices will enter an invalid state
and, on top of that, Apple's Bluetooth Low Energy guidelines explicitly
discourage this behavior.

In order to allow interoperability with those devices, make the GATT
automatic security request sending as a peripheral optional by
introducing a new Kconfig option, BT_GATT_AUTO_SEC_REQ.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>