Add placeholder square for screenshots in install dialog

Previously before the screenshots loaded, there was a big gap at the
bottom of the dialog, then the buttons would jump to the bottom to fit
the screenshots in.
Now there is a gray placeholder which is exactly where the screenshot
goes when it loads.

Before: https://b.corp.google.com/action/issues/343106164/attachments/56798352?download=false
After: https://b.corp.google.com/action/issues/343106164/attachments/56798395?download=false

Bug: b/343106164
Change-Id: I18f1b29e480cb2a556c8324c6d64c382996cd52b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5568446
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Louise Brett <loubrett@google.com>
Cr-Commit-Position: refs/heads/main@{#1308476}