Fix artist items continuation

[gechoto]

These changes make it possible to load:

• more than 100 songs on an artists page
• more than 100 albums on an artists page

Also it removes duplicate code in ArtistItemsContinuationPage.

There is still room for optimization. For example in ArtistItemsScreen we now always call two LaunchedEffect (for lazyListState and lazyGridState).
I don't know how much worse this is for performance.
We could do this conditionally but since I'm still learning and never used Compose before I'm not feeling confident yet doing changes in this part of the code.
Maybe someone else can work on that or I might open a separate draft PR for optimizations.

Also the initial loading placeholders are still not correct. For example when loading albums (displayed in a grid) it still initially shows list item placeholders (because it doesn't know yet that it should display a grid).
Looks a bit weird. But nothing new, this already happened before this PR so I would not change it here. Maybe something for later.

[gechoto]

Hi @z-huang I think this is ready for merge.
I have been using it since I made this PR and haven't found any problems with it.

[z-huang]

Thanks!