Fix hotwording's use of a deprecated function.
Fix hotwording's use of a deprecated function.

MediaStream.stop() has been removed in M47.

BUG=538894

Review URL: https://codereview.chromium.org/1389263003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353487}

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