[Android] Make keyboard accessory views independent from BottomContainer

With this CL, the view stubs (and the views) are moved out of the
bottom container, which makes it possible (and necessary) to push up
all visible content.

This fixes:
 - the accessory overlaying page contents (esp. bad for fullscreen)
 - the accessory being pushed up by the keyboard (visual flickering)
 - clicking through the accessory

This changes:
 - the bar isn't pushed up by bottom sheets (like download) anymore BUT
   instead pushes them up when the bottom sheet is opened

Bug: 874257, 873591, 704070, 853745, 865964
Change-Id: I8185b1ac21aaaed3d7123855b8964a3f86c9845a
Reviewed-on: https://chromium-review.googlesource.com/1177361
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Vaclav Brozek <vabr@chromium.org>
Commit-Queue: Friedrich Horschig <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583987}
19 files changed