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Adds expansion from default view to expanded view with apps list. This
CL also moves Sharesheet UI classes into their own folder.

Bug:1130406

Change-Id: I17824948bdb20a5e3cfa6b7ac93b0216ee43cf61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2409480
Commit-Queue: Melissa Zhang <melzhang@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809216}