[Promise Icons] Download icons and add to PromiseAppIconCache

Create PromiseAppIconCache class to store the icons of promise apps.
After we receive a response from Almanac, use the icon URLs to download
the icons and save them into the PromiseAppIconCache.

Bug: b/261910028
Change-Id: I7d22d00e452b047141576cb778e91529a247c8ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4522697
Commit-Queue: Vivian Pao <vpao@google.com>
Reviewed-by: Maggie Cai <mxcai@chromium.org>
Reviewed-by: Ehimare Okoyomon <eokoyomon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148885}