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By limiting Assistant UI size not to exceed the usable work area:

-Fix Assistant UI goes offscreen if there is not enough screen space
 left when soft keyboard is present or rotate/zoom in the display.
-Fix Assistant UI disappears off the screen to the top during resizing
 animation with soft keyboard shows.
-Fix Assistant UI container resizes to the wrong size when dismissing
 soft keyboard.

Test: local compile and manually test.
Bug: b:111444849, b:112927824
Change-Id: I4a0c641386309f857ffd8ddc2b5a73187cc807c5
Reviewed-on: https://chromium-review.googlesource.com/1226369
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594802}