Add read later empty state image

Accounts for light and dark mode, as well as both bubble and side panel
containers.

Bug: 1230742
Change-Id: I04b838a7bf9e1b51e6627f62f6c06c34fec39110
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3042199


Commit-Queue: Emily Shack <emshack@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/master@{#906346}