Bluetooth: Mesh: Calculate adv duration instead of using estimation

The duration value passed to `struct bt_mesh_adv_cb::start` is an
estimation.

When running mesh with BT_MESH_ADV_EXT option, the actual duration will
be shorter and the `struct bt_mesh_adv_cb::end` is called right after
the contorller finished advertising. Using the calculated duration the
lpn node can be configured for a shorter receive delay and receive
window and eventually use less power.

When running mesh with BT_MESH_ADV_LEGACY option, the time when adv will
be sent out by controller is unpredictable. But the time between start
and end `struct bt_mesh_adv_cb` callbacks is always 50ms for a single
advertisement. Therefore, the receive delay can't be less than 50ms.

With this change, adv duration is calculated between start and end
callbacks, but timers are started after end callback is called. Therefore
we need to consider time between two callbacks when setting timeouts for
timers.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>