FLUT-277: Updated to focus_detector_v2 so that latest version of visibility_detector is used

[ygit]

Description

• Closes Update internal library "visibility_detector": to latest version 🐛  #1712
• Removed old focus_detector package which was not updated since 2 years
• Updated to focus_detector_v2 so that latest version of visibility_detector is used

Pre-launch Checklist

• The Documentation is updated accordingly, or this PR doesn't require it.
• The ExampleAppChangelog is updated with related tickets, or this PR doesn't require it.
• I updated/added relevant documentation (doc comments with ///).
• I listed at least one issue that this PR fixes in the description above.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.