Add AssistantSuggestion support for local icons.

Previously, AssistantSuggestions only supported remote icons.
Now, they will optionally support local icons as well. These will be
specified by use of resource links (a similar concept to deep links
which instead of pointing to some action to be performed point to some
resource to be used).

This will be utilized in a follow up CL to provide local icons to
Assistant onboarding suggestions.

Bug: b:157510970
Change-Id: I488865044c1485b1e431cd843f2d0ea0ad146be5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2289074
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#787920}