Add a field to differentiate between dragged apps and folders

Folders are only supposed to be received by the AppsGrid.  Create a new
data structure to hold information about the dragged app.  The structure
will be written into a dat Pickle to be transported within the
OSExchangeData.  DraggableAppItemInfo will hold the app id and the
element type (folder or app) that is attempting to be dropped.

Bug: b:283125386
Change-Id: I5c3eb9c74e9e1f9cf390b12855e4e587fd7bc765
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4546031


Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Ana Salazar <anasalazar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147375}