Bluetooth: Mesh: advertiser: add disable function

Adds a disable function for the extended advertising which stops and
deletes the advertising instances, allowing them to be properly
reinitialized when calling `bt_mesh_adv_enable()` after bluetooth has
been disabled and re-enabled.

For the legacy advertising, the function terminates the advertising
thread. If legacy advertising is used, `bt_mesh_adv_init()` must be
called before `bt_mesh_adv_enable()` to properly resume advertising
after suspension.

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