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There are some cases that one task exposes multiple accessibility
windows in Android. After this change, non-root windows have a generic
container role instead of an application role so that while moving focus
between windows, ChromeVox won't announce entering or exiting the
application.

Also, we won't dispatch focus for non-focused windows. Each window has
a focused node, but previously all focus events are handled equally.
With this change, only the focused window is considered in the focus
handling logic.

AX-Relnotes: n/a.
Bug: b:150827488
Test: current unit tests pass.
Change-Id: Icd900ef1dcf84926240c6b55f000ba75a05e1ad8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2576227
Commit-Queue: Hiroki Sato <hirokisato@chromium.org>
Reviewed-by: Sara Kato <sarakato@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836987}