fix(privacy): Fix shred button visibility

[StephenHeaps]

• Hide shred button when showing 0 tabs in private mode
• Hide shred button when showing tabs from other devices

Resolves brave/brave-browser#41671

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. Enable Shred feature flag (#brave-shred) in brave://flags, quit Brave from app switcher & relaunch Brave.
2. In 'Settings' -> 'Sync', setup sync if not setup and enable 'Open Tabs' under 'Sync Settings'.
3. Verify Shred button is visible when in tab tray in regular mode.
4. Verify Shred button is hidden when viewing 'Tabs from other devices' in tab tray.
   • When syncing Open Tabs, open tab tray and switch to this view in regular mode via the top control
5. Verify Shred button is hidden when in tab tray in private mode with 0 tabs
6. Verify Shred button is visible when in tab tray in private mode with 1+ tabs