Announce timeline jump-to-bottom to TalkBack

[unclejay80]

Content

The "jump to bottom" action scrolls the timeline to the newest message (lazyListState.scrollToItem(0)) but never announces that the jump happened, so TalkBack users get no confirmation the scroll occurred.

Motivation and context

Announce completion of the jump via View.announceForAccessibility() once the scroll animation finishes.

Screenshots / GIFs

No visual change — the fix only announces once the existing scroll animation completes; the scroll behaviour and layout are unchanged. TimelineView.kt's existing @PreviewsDayNight previews already cover its appearance.

Tests

• Step 1: scroll up in a chat, trigger jump-to-bottom, with TalkBack confirm the jump is announced
• Step 2: verify with TalkBack enabled

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• I am aware of the etiquette.
• This PR was made with the help of AI:
  • Yes. Please request a review by Copilot.
  • No.
• Changes have been tested on an Android device or Android emulator with API 24.
• UI change has been tested on both light and dark themes.
• Accessibility has been taken into account.
• Pull request is based on the develop branch.
• Pull request title will be used in the release note, it clearly defines what will change for the user.
• Pull request includes screenshots or videos if containing UI changes.
• You've made a self review of your PR.

[github-actions]

Thank you for your contribution! Here are a few things to check in the PR to ensure it's reviewed as quickly as possible:

• If your pull request adds a feature or modifies the UI, this should have an equivalent pull request in the Element X iOS repo unless it only affects an Android-only behaviour or is behind a disabled feature flag, since we need parity in both clients to consider a feature done. It will also need to be approved by our product and design teams before being merged, so it's usually a good idea to discuss the changes in a Github issue first and then start working on them once the approach has been validated.
• Your branch should be based on origin/develop, at least when it was created.
• The title of the PR will be used for release notes, so it needs to describe the change visible to the user.
• The test pass locally running ./gradlew test.
• The code quality check suite pass locally running ./gradlew runQualityChecks.
• If you modified anything related to the UI, including previews, you'll have to run the Record screenshots GH action in your forked repo: that will generate compatible new screenshots. However, given Github Actions limitations, it will prevent the CI from running temporarily, until you upload a new commit after that one. To do so, just pull the latest changes and push an empty commit.