Issue 7170: Make ThumbnailsUseCases optional in TabsFeature and TabsTrayPresenter

[gabrielluong]

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Fixes #7170. This also rolls back the changes to the TabsTrayPresenterTest from #7021, which are not necessary if we made ThumbnailsUseCases optional.

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry or does not need one
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

After merge

• Milestone: Make sure issues closed by this pull request are added to the milestone of the version currently in development.
• Breaking Changes: If this is a breaking change, please push a draft PR on Reference Browser to address the breaking issues.

[codecov]

Codecov Report

Merging #7171 into master will decrease coverage by 0.33%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #7171      +/-   ##
============================================
- Coverage     77.26%   76.92%   -0.34%     
+ Complexity     5048     4890     -158     
============================================
  Files           675      662      -13     
  Lines         24707    23858     -849     
  Branches       3641     3490     -151     
============================================
- Hits          19090    18353     -737     
+ Misses         4111     4045      -66     
+ Partials       1506     1460      -46     

Impacted Files	Coverage Δ	Complexity Δ
...la/components/feature/tabs/tabstray/TabsFeature.kt	90.90% <0.00%> (-4.33%)	7.00 <0.00> (ø)
...ponents/feature/tabs/tabstray/TabsTrayPresenter.kt	94.28% <100.00%> (+0.16%)	10.00 <1.00> (+1.00)
.../feature/downloads/AbstractFetchDownloadService.kt	68.00% <0.00%> (-1.10%)	63.00% <0.00%> (ø%)
...ents/support/migration/MigrationIntentProcessor.kt	100.00% <0.00%> (ø)	3.00% <0.00%> (ø%)
...onents/support/sync/telemetry/BaseGleanSyncPing.kt	100.00% <0.00%> (ø)	11.00% <0.00%> (ø%)
...components/lib/jexl/evaluator/EvaluatorHandlers.kt
.../src/main/java/mozilla/components/lib/jexl/Jexl.kt
...in/java/mozilla/components/lib/jexl/lexer/Token.kt
.../mozilla/components/lib/jexl/ext/JexlExtensions.kt
...mozilla/components/lib/jexl/evaluator/Evaluator.kt
... and 8 more

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[jonalmeida]

bors r+

[bors]

Build succeeded:

• complete-push