[PWA] Load all screenshots unconditionally for detailed dialog

This CL does the following:
1. Constructs the number of spaces in the detailed dialog based not
on the number of screenshots available in the manifest, but on the
number of screenshots to be downloaded.
2. Ensures that all screenshots are loaded unconditionally, except for
if the bitmap does not draw anything. This is needed because empty
bitmaps would crash the ImageModel [1] on debug enabled builds.

https://source.chromium.org/chromium/chromium/src/+/main:ui/base/models/image_model.cc;l=153?q=ImageModel::GetImage

Tested using https://cute-western-seer.glitch.me/

Fixed: 390075225
Bug: 372285217
Change-Id: I4b3f6db586711e0d85ada8a9469ff0e2f4ad8ea5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6176912
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Dibyajyoti Pal <dibyapal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1406849}