Bluetooth: Mesh: Fix proxy advertiser handling with GATT server enabled

When the GATT server is enabled the proxy advertiser
server need to be stopped. This solved a bug where the
Mesh Proxy Service is not restared.
This fix is done to fix several failing PTS test that
was failing when CONFIG_BT_MESH_ADV_EXT_GATT_SEPARATE
was enabled. Calling the stop_proxy_adv was left out
in a prevouis commit when doing code cleanup.

Signed-off-by: Ingar Kulbrandstad <ingar.kulbrandstad@nordicsemi.no>