wallpapers/image: fix random order update logic

In SlideFilterModel, an added/deleted wallpaper is not always the last
one, so `first` and `last` also needs to be taken into account.

`m_randomOrder` stores the row numbers of the source model in a specific order.
 When a new wallpaper is added, the `first` argument is not always equal
to `sourceModel->rowCount()`, and when the condition is not met, all the
existing row numbers in `m_randomOrder` that >= `first` need to be
increased by 1 (or the number of new wallpapers), to make space for new
wallpapers. Vice versa when wallpapers are removed.

CCBUG: 473088
(cherry picked from commit 79f494d6)