adb: fix double close in wait-for-*.

unique_fd's implicit conversion to int allows the following code to
compile without error, leading to a double close:

  std::function<void(unique_fd)> func = [](int x) { close(x); };
  func(unique_fd(42));

Test: treehugger
Change-Id: I948ecda3a12738b3af6444fe2902d2f7b80e1b4c