Fix Tweener leak on Animation

[pictos]

Description of Change

This PR is a piece of #25232, that is a WiP to fix memories leaks on Animation.

This PR prevents the Tweener to live forever and the s_tweeners collection to grow forever.

Issues Fixed

Fixes #25001

[dotnet-policy-service]

Hey there @pictos! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[jsuarezruiz]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[rmarinho]

Can we port the unit test to this PR too ?

[pictos]

@rmarinho the unit tests aren't related to this change. The s_tweeners is private and there's no public/internal member that exposes its Count property, I can't see a way to test but always open to ideas.

The tests in previous PR are related to Animation leak object, and it's possible to test because we can see the leak in public members or the control that implements the animation

[rmarinho]

Can we add a test to check the public Tweener leaks or not ? do we make s_tweeners internal ?

[pictos]

@rmarinho I added a property to expose the s_tweeners.Count as discussed with @PureWeen, so I believe it's good to go

Tests added