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The hotwording checkbox was moved from the 'Privacy' section
of the settings page in https://codereview.chromium.org/382123004/

This cl moves it back to the 'Privacy' section and shows and hides
the 'Voice' section according to the enable-experimental-hotwording flag.

BUG=407893

Review URL: https://codereview.chromium.org/523573004

Cr-Commit-Position: refs/heads/master@{#292857}