Copies android behaviour. Once an app is snapped, the remaining overview
items which cannot be snapped show a message saying they cannot be
snapped.

Also show a toast when user tries to enter splitview while a cannot
snap app is currently active. This can happen if users select a cannot
snap app when there is a snapped app already, or by longpress on the
overview button if the current app is not snappable.

Pulls RoundedRectView out of window grid since it is the same as
RoundedContainerView and a similar class is needed to display the cannot
snap message.

Test: manual
Bug: 774238
Change-Id: Ice993ae94ae5495292c179e0b5bf9ef97d3f8c1c
Reviewed-on: https://chromium-review.googlesource.com/722028


Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513668}