Bluetooth: Mesh: Fix gatt advertiser start in bt_mesh_resume

To signal to the mesh extended advertiser that a proxy advertisement is
started, `bt_mesh_adv_gatt_send()` is called in combination with
setting the `ADV_FLAG_PROXY` flag in adv_ext.c. This ensures that it
won't try to start the advertiser again, which would result in the
controller rejecting it.

Setting this flag outside adv_ext.c is not viable, and as such calling
`bt_mesh_adv_gatt_send()` should also be limited to the advertising
source code.

As it stand now, once we utilize the new commit priority for `h_commit`
calls such a recall of `bt_mesh_adv_gatt_send()` with following
rejection by the controller will happen.

Within the `bt_mesh_resume()` function we can confidently assume the
extended advertiser has already been started. As such we can call
`bt_mesh_adv_gatt_update()` instead of `bt_mesh_adv_gatt_send()`.

This change has been tested with both running relevant bsim tests after
rearranging in which order the settings of the subsystems are loaded via
`h_commit`, as well as a modified light switch sample, adding
`bt_mesh_resume/suspend` calls on buttons.

Signed-off-by: Kyra Lengfeld <kyra.lengfeld@nordicsemi.no>