Bluetooth: Mesh: suspend/resume gatt advs

Disables pb gatt- and gatt proxy advs when suspending Mesh, and
enables them again when resuming Mesh.

Adds `bt_mesh_adv_gatt_send` to `bt_mesh_resume` to make sure that
GATT advs start after resumption.

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>