Bluetooth: Use helper function for re-enabling advertising

When the all LE connections have been disconneted, then it is up to
the host to re-enable advertising at that point. To ensure that the
correct advertising parameters are used, force the usage of the
common helper to enable advertising.

The change just moves the manual enabling of advertising from the
event handler into the management core so that the helper can
be actually shared.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>