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This change:
- adds test coverage for Learn Mode with regard to keyboard, gesture, and braille
- adds live descriptions of each gesture (e.g. swipe right with one finger) in addition to describing the command
- adds the touchExplore gesture as something described (with special handling due to how frequently it fires)
- is more careful about queueing up too much speech; added robust tests for this throughout

R=akihiroota@chromium.org

Fixed: 1106462
AX-Relnotes: only announce a modifier once, when holding down modifiers like Search in ChromeVox Learn Mode. Also, adds descriptions for gestures like swipe right with one finger, in addition to the command triggered while in Learn Mode.
Change-Id: Ib600330c63cf3c412aa62c34050a4281e07cb816
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2309981
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790591}