First problem is even searching in the directories that flatpaks may use. Currently we hardcode the /usr/share directory to look in unless XDG_DATA_DIRS is set, in which case we use the paths in there. So probably the better way to do this is to update the env var to be this:

XDG_DATA_DIRS=$HOME/.local/share:$HOME/.local/share/flatpak/exports/share:/usr/share:/var/lib/flatpak/exports/share

Then we cover local overrides for users first and also handle where flatpak puts stuff.  After we do that, then we can see if the GIMP icon will show up or not for the flatpak app...and if it doesn't we can address that issue.

Temporarily assigning to smbarber for implementing the env var...then assign back to me after that's done.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/0668f9243fd36ec3428c7fbd8a52dc2aaa4d6503

commit 0668f9243fd36ec3428c7fbd8a52dc2aaa4d6503
Author: Jeffrey Kardatzke <jkardatzke@google.com>
Date: Fri May 18 05:21:11 2018

vm_tools: Fixed bug where icons didn't use XDG_DATA_DIRS

When testing this I also applied a patch that I'll upload shortly which
updates XDG_DATA_DIRS for garcon.

BUG= chromium:839163 
TEST=Verified icons in alternate dirs show up

Change-Id: I1196e7606aa51cefc2627e1a3f3dff04f239af2e
Reviewed-on: https://chromium-review.googlesource.com/1062788
Commit-Ready: Jeffrey Kardatzke <jkardatzke@google.com>
Tested-by: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-by: Tim Zheng <timzheng@chromium.org>

[modify] https://crrev.com/0668f9243fd36ec3428c7fbd8a52dc2aaa4d6503/vm_tools/garcon/desktop_file.cc
[modify] https://crrev.com/0668f9243fd36ec3428c7fbd8a52dc2aaa4d6503/vm_tools/garcon/icon_finder.cc