fix(reviewer): replace AlphaAnimation with ViewPropertyanimator in AnswerFeedbackView

[SajalDevX]

Purpose / Description

Fixes a dark smudge/visual artifact at the top edge of card content when flipping cards in the new study screen .
The artifact appears when all the following conditions are met:

• Dark mode (night theme) enabled
• "Show answer feedback" enabled (Settings > Study Screen)
• Frame style set to "Box" (Settings > Study Screen > Frame style)
• Cards use custom CSS with "backdrop-filter" / translucent background

Fixes

• Fixes Dark smudge artifact at top of card content when flipping (New study screen, dark mode, "Show answer feedback" + Frame style "Box") #21380

Approach

• AlphaAnimation puts the view on a software layer, but the WebView's backdrop-filter uses a hardware backdrop root layer. Normally the card's elevation/rounded corners hide the seam, but Frame style "Box" turns both off (PreviewerHelpers.kt:setFrameStyle()), so it shows.
• Switched to ViewPropertyAnimator hardware accelerated, composites correctly with the backdrop layer. Same timing/interpolators, no visual change otherwise.
• Also removed the two now unused XML animation files.

How Has This Been Tested?

• Unit tests have been run and verified
• Manually tested the fix with the version before and after on 2 devices -> (VivoV27-15) (Pixel-8a-17-emulator)

Learning (optional, can help others)

• "ViewPropertyAnimator" is hardware-accelerated and participates in the normal layer
• "android.view.animation.Animation" forces a software layer on the view
• WebView with "backdrop-filter" creates a hardware layer that captures what's behind it for the blur effect

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[welcome]

First PR! 🚀 We sincerely appreciate that you have taken the time to propose a change to AnkiDroid! Please have patience with us as we are all volunteers - we will get to this as soon as possible.

[BrayanDSO]

Have you been able to reproduce the original issue?

[SajalDevX]

Have you been able to reproduce the original issue?

yes

[SajalDevX]

@BrayanDSO Check this out and please let me know if you need help in reproducing this issue.
https://github.com/user-attachments/assets/f95c0fe1-668c-4143-bca6-18be3dc35d70