focus_mode: Add support for track thumbnails

The image is downloaded in native code using the focus mode traffic
annotation. It is then encoded as a data URL and passed to the player.

BUG=b:337064323

Change-Id: I31b286aea394217a8034a360047966728a41809e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5609712


Reviewed-by: Richard Chui <richui@chromium.org>
Commit-Queue: Richard Chui <richui@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312368}