adb: detect when the client disconnects in wait-for-device.

Avoid leaking a thread and its associated resources when a user cancels
wait-for-device.

Bug: http://b/26966721
Bug: https://code.google.com/p/android/issues/detail?id=199088
Change-Id: Idac80a24e9739ddd24e500fe14826a78f350c018
(cherry picked from commit 09855472)