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Prior to this CL, clicking cancel on the connecting dialog simply
returns the user to the home screen, without disconnecting the session or
cleaning up any related resources.

This CL provides a cleaner way to cancel a session with by implementing
remoting.ConnectingDialog which calls stop() on the activity.

BUG=477522

Review URL: https://codereview.chromium.org/1101613003

Cr-Commit-Position: refs/heads/master@{#326926}