adb start-server: Use a separate fd for sending initial OK

When "adb start-server" is issued, and a server needs to be launched,
adb client forks itself and the child process runs the server routine.
Once the server initializes its various components, it sends an "OK\n"
back to the client via its stderror (or stdout on Windows).

This sequence breaks down if before sending the "OK\n", the server
happens to log something on its stderr. In order to avoid this, the
client now expects the ack to come on a different fd rather than one
of the standard streams.

Bug: https://code.google.com/p/android/issues/detail?id=182150

Change-Id: I9d58a08068d71eb3b77e8a7377e934631c016466