Bluetooth: host: invalidate the RPA when starting legacy adv

This fixes the failure to use a resolvable private address in this
scenario.

1. call `bt_le_oob_get_local`, will generate and mark RPA as valid
2. start connectable adv w/ IDENTITY bit
3. start connectable adv w/o IDENTITY
4. RPA is not set (in `bt_id_set_private_addr`) because RPA
   is still marked as valid

When EXT_ADV is enabled and the controller supports it, a different code
path is taken that doesn't have this issue.

Unconditionally invalidating the RPA when starting advertising works around
this issue.

Fixes #56326

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