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In order to show notifications about blocked USB devices when
appropriate, ash needs to be aware of these events. This CL adds the
logic necessary to observe the appropriate D-Bus signal.

Bug: 987351
Change-Id: I3eacbc3c401ba2f7d6dc0272a0b56c689ac8cd5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1772369Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Allen Webb <allenwebb@google.com>
Cr-Commit-Position: refs/heads/master@{#698936}