[fuchsia] Explicitly handle unsupported a11y actions

For actions that are not supported on the Fuchsia SemanticsManager side,
handle those explicitly by running
OnAccessibilityActionRequestedCallback with false. This indicates the
action was received and deliberately not handled.

Add ConvertAction to convert a single Fuchsia action to a Chromium
action.

Test: Added AccessibilityBridgeTest.PerformUnsupportedAction
Bug: fuchsia:16501
Bug: 973095
Change-Id: I32fc33b439ae7cca951c21fcf6d875a07c8fa76c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144343
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758338}