Refactor #getThumbnailPlaceholderIconColor.

Multiple minor stylistic changes to the method. no functional changes.
* Moved default_icon_color_on_accent_1 into SemanticColorUtils.
* Replaced custom applyAlpha with a ColorUtils call.
* Adjusted variables names to self document and removed all comments.
* Added final to local variables to inline annotation.

Change-Id: If4ddacdcbc15c49726016eebb881e86efd6cf2dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4616018
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Commit-Queue: Sky Malice <skym@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1158283}