Issue #6827: Move Thumbnails into its own browser component

[gabrielluong]

Fixes #6827.

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As we work through adding a disk cache for thumbnails in #2754, it is becoming clear we should move the thumbnails feature into its own browser component. This is the first step in doing that by copying over ThumbnailsFeature.kt and ThumbnailsFeatureTest.kt over to BrowserThumbnails.

We also switched the Sample Browser to use the new BrowserThumbnails component. I have categorized this component as under development since #2754 will make it more of a real feature.

As the next step that I want to do is look into adding a deprecation warning for ThumbnailsFeature, but I didn't think it was necessary in this PR and could be made as a follow up.

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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 #6835 into master will increase coverage by 0.15%.
The diff coverage is 78.57%.

@@             Coverage Diff              @@
##             master    #6835      +/-   ##
============================================
+ Coverage     77.42%   77.58%   +0.15%     
- Complexity     4549     4764     +215     
============================================
  Files           607      632      +25     
  Lines         22671    23254     +583     
  Branches       3343     3424      +81     
============================================
+ Hits          17554    18042     +488     
- Misses         3717     3783      +66     
- Partials       1400     1429      +29     

Impacted Files	Coverage Δ	Complexity Δ
...components/browser/thumbnails/BrowserThumbnails.kt	78.57% <78.57%> (ø)	8.00 <8.00> (?)
...ents/browser/domains/DomainAutoCompleteProvider.kt	65.78% <0.00%> (ø)	12.00% <0.00%> (?%)
...mponents/browser/domains/autocomplete/Providers.kt	100.00% <0.00%> (ø)	0.00% <0.00%> (?%)
.../java/mozilla/components/browser/domains/Domain.kt	63.63% <0.00%> (ø)	4.00% <0.00%> (?%)
...omponents/service/fretboard/ExperimentsSnapshot.kt	100.00% <0.00%> (ø)	3.00% <0.00%> (?%)
...retboard/storage/flatfile/ExperimentsSerializer.kt	100.00% <0.00%> (ø)	6.00% <0.00%> (?%)
...mozilla/components/service/fretboard/Experiment.kt	100.00% <0.00%> (ø)	14.00% <0.00%> (?%)
...lla/components/service/fretboard/ValuesProvider.kt	100.00% <0.00%> (ø)	10.00% <0.00%> (?%)
.../components/service/fretboard/ExperimentPayload.kt	100.00% <0.00%> (ø)	9.00% <0.00%> (?%)
... and 16 more

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[jonalmeida]

This looks good to me! Please put up a PR for r-b to fix the breaking APIs for when they land.

(land if CI is happy)

[gabrielluong]

bors r+

[bors]

Build succeeded:

• complete-push