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This change adds support of Android AutoCompleteTextView for Chrome
automation, so that ChromeVox can handle it and make an announcement.

See go/arc-autocomplete-a11y for details.

This also changes ChromeVox's announcement on active descendant changed
events, so that it won't announce hints as the hints are mostly for the
focused node.

AX-Relnotes: Adding a support of Android app's autocomplete of editables.
Bug: b:150827488
Test: unit_tests

Change-Id: I2f2dd9832311a080d24f1e0771b2e97ecd455e5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2576068
Commit-Queue: Hiroki Sato <hirokisato@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Sara Kato <sarakato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837007}