[PWA/Trusted] Return purpose when icon manager reads trusted icons

This CL updates the behavior of
WebAppIconManager::ReadTrustedIconsWithFallbackToManifestIcons() in
a few ways to support returning IconPurpose as part of the callback
that it takes in. To implement this, a few more things have been
fixed:

1. The current implementation is more robust. The previous
   implementation would iterate per size, and there was a chance
   that it could return a MASKABLE icon of size #1, but an ANY icon
   of size #2. That is inconsistent with the previous
   implementation, where if a size for a particular purpose was
   found, it would be returned, else the size entry for that
   purpose would stay empty. This is done by hooking up the
   implementation to the internal ReadIconsBlocking() API, used
   by the previous implementation, thus streamlining the
   behavior.
2. A callback wrapper is introduced for callsites that DOES NOT
   want the IconPurpose information and only wants the bitmaps.
   This made migrating the callsites much easier.
3. ReadUntrustedIcons() is now a private API. The unit-tests using
   that API have been kept and friended as per Chromium style
   guides.

Use the returned IconPurpose on production in the uninstall
dialog delegate view to determine if the icons need to be masked
or not.

Change-Id: I630ad07c6353532772852261d62e95abb3b127a4
Bug: 427566193
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6915967
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Dibyajyoti Pal <dibyapal@chromium.org>
Reviewed-by: May Siem <msiem@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1512542}