Refactor ItemsLayout handling: dynamic default + virtual view-managed subscriptions

[bhavanesh2001]

Note

Are you waiting for the changes in this PR to be merged?
It would be very helpful if you could test the resulting artifacts from this PR and let us know in a comment if this change resolves your issue. Thank you!

Description of Change

This PR resolves #29619 by eliminating the memory leak caused by CollectionViewHandler2's subscription to ItemsLayout property changes and reworking the property change flow.

Alternative to #29635 , #28675

Problems

• Previously, we used a static LinearItemsLayout.Vertical instance as the default for ItemsLayout
• When the handler subscribed to property changes on this shared instance (without unsubscribing), it caused memory leaks (in the CV2 case)
• Additionally, Android and Windows used WeakNotifyPropertyChangedProxy to work around this, but that added platform complexity and only masked the root cause.

Fix

• The default ItemsLayout is now created per-instance using defaultValueCreator, not a static object.
• The ItemsView (virtual view) subscribes directly to changes on its ItemsLayout and raises OnPropertyChanged("ItemsLayout") for relevant internal changes (Span, ItemSpacing, etc.).
• This triggers the MapItemsLayout mapper, which applies platform-specific updates.
• Removed WeakNotifyPropertyChangedProxy from Android and Windows for ItemsLayout changes.
• All layout updates now flow through the mapper via the virtual view

Issues Fixed

Fixes #29619
Fixes #27666
Fixes #27667
Fixes #28656
Fixes #29696

[rmarinho]

Can we add the CV2 to the leak tests ?

[bhavanesh2001]

While enabling memory tests for CV2, I found that CarouselViewHandler2 is leaking 😢. It seems the cause is different from the issue we're addressing here — something else is triggering the leak.

Anyway, I’ll definitely look into it.

Aside from that, everything here looks great.

[rmarinho]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[bhavanesh2001]

Only CarouselViewHandler2 memory tests are failing. #29719 should fix the leak.

I also noticed that CarouselView too uses a static instance as the default ItemsLayout. I'm updating it to use a per-instance value via defaultValueCreator and managing property changes in the VirtualView itself—just like we did with ItemsView for CollectionView.

Also, I need to write few UI tests to verify that property changes in ItemsLayout (e.g., ItemSpacing) are correctly reflected in the UI for both CollectionView and CarouselView

[rmarinho]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[bhavanesh2001]

@rmarinho, shouldn't the declaring type here be typeof(CarouselView)?

[jsuarezruiz]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).