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Disable the "Stop" button after it's clicked, so that the user doesn't
click on it repeatedly. The button becomes enabled again when there is
a model update, or if the user clicks on a sink.

When the stop button is clicked for a sink, the sink enters DISCONNECTING
state for that dialog, until its route is removed.

Bug: 848975
Change-Id: I0acde957053a445fce6bb24cdb54c5e30dd45587
Reviewed-on: https://chromium-review.googlesource.com/1123326
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577665}