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This is the first step in a series of changes to improve touch interaction with ChromeVox.

This change:
- introduces a new earcon when entering a touch anchor
- introduces a new earcon when touching a blank area of the screen (defined as an anchor that is neither a touch leaf nor a touch object)
- when moving from a touch anchor to a non-touch anchor, immediately stop speech so the user knows they have left. This will help avoid confusion about what's currently focused or not when hovering a finger. This might also be familiar (e.g. iOS behaves this way).

Note for the last point, we will have to solve the issue of really small touch targets. One potential solution (not for this change) is to come up with a way to do "fuzzy" hit testing.

Change-Id: I262c664bc0d86aa6e8ce96cf1e263db7a8f184cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088510
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747708}