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Moving forward, when an add or removal of a holding space item occurs,
the current container contents will animate out after which the
container will be updated and its new contents animated in.

This CL does not implement the animations but stubs in places for them.
There should be no user visible change as a result of this CL.

Note that this CL also attempts to simplify logic by removing all
holding space item views when they are animated out and re-adding any
views necessary before animating them in. This is slightly less
performant but since the view count is small, the code is much simpler
than would otherwise be if needing to diff the view state with the model
state.

Bug: 1154998
Change-Id: I822d1a6e4caadd1ddc6cc008429bfd1b28a0bb9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2572056Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#833629}