Bluetooth: Host: Fix not clearing IDs and keys upon bt_disable()

Expectation: After calling `bt_disable()` it is possible to
use the Bluetooth APIs as if `bt_enable()` was never called.

This was not the case for `bt_id_create()`, it was not possible
to set the default identity. This prevented an application
developer to restart the stack as a different identity.

Keys also need to be cleared to avoid the following pattern:
1. Pair two devices
2. Central calls `bt_disable()` and `bt_enable()`.
   The central will now generate a new identity address.
3. Connect the two devices.
4. Re-establish encryption. Now the central will try to use
   the previously used keys. The procedure will fail
   because the peripheral does not have any keys associated
   with the new central address.

The API documentation is updated accordingly.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>