[Android TRM] Fold ThumbnailProvider into UrlImageProvider.

TabResumptionTileContainerView.loadLocalTabSingle() uses
ThumbnailProvider.getTabThumbnailWithCallback() to get a thumbnail.
Previously there was much plumbing to get ThumbnailProvider to where it
gets used, including storing it as property THUMBNAIL_PROVIDER.

However, UrlImageProvider (property URL_IMAGE_PROVIDER) already provides
other images: {favicon, large icon, fallbacks}. Although thumbnails are
keyed by tab ID and not URL, getting UrlImageProvider to provide these
is not a large departure, and can reduce plumbing.

This CL folds ThumbnailProvider into UrlImageProvider to simplify code,
and applies adjacent restructuring.

Details:
* Remove ThumbnailProvider plumbing: Builder -> Coordinator -> Mediator
  -> Property THUMBNAIL_PROVIDER -> Binder -> View -> ContainerView.
* UrlImageProvider.UrlImageSource:
  * Add createThumbnailProvider().
  * Move implementation to new file UrlImageSourceImpl.java.
    * createThumbnailProvider(): Take from Builder.
* UrlImageProvider:
  * Remove inline instantiation of UrlImageSource; update Builder to
    instantiate UrlImageSourceImpl and pass into UrlImageProvider.
  * Add getTabThumbnail(), with param simplified by passing default
    values to getTabThumbnailWithCallback().

Unit tests:
* TabResumptionModuleSuggestionsUnitTest.testUrlImageProvider(): Add
  cases for thumbnails. Using doAnswer() to inject expected thumbnail.
* Add UrlImageSourceImplUnitTest with 1 cursory test for coverage.

Bug: 339269597
Change-Id: I401976dfb50195b46d5ae31ebc55c92fbdf8f6cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5544868
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1305141}