fix: avoid anchoring reviewer snackbar to hidden answer buttons

[criticalAY]

Purpose / Description

In FullScreenMode.FULLSCREEN_ALL_GONE, the answer buttons layout is animated to View.GONE when the system UI auto-hides. Snackbars shown during that window (e.g. via undoAndShowSnackbar) anchored to the hidden view and tripped the visibility check in showSnackbar. Guard the anchor with takeIf { it.isVisible }, mirroring the fixapplied to DeckPicker in #20766.

Fixes

• Fixes [BUG] 2.24: undoAndShowSnackbar - anchorView was not visible #20946

Approach

NA

How Has This Been Tested?

Google emulator API 35, I have verified it doesnt occur in new study screen only in old one

Learning (optional, can help others)

NA

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

[criticalAY]

@david-allison AbstractFlashcardViewer.kt will be gone when we finally enable new SS and get rid of this one (No action needed)