adb: Don't synchronize with transport using sleep

Services that cause adbd to reboot are currently implemented this way:
    write(fd, response)
    close(fd)
    sleep(1)
    exit(1)
sleep(1) is necessary to leave time for the transport to transmit
the response before the daemon exits. This is slow and unreliable.
Instead, add a flag to the asocket to make it exit the daemon only
after the socket is closed.

Change-Id: I9df45ea6221f0d9b828703b9b2316a5d4fe59352
diff --git a/adb/adb.h b/adb/adb.h
index 85922bf..03a7393 100644
--- a/adb/adb.h
+++ b/adb/adb.h
@@ -86,6 +86,11 @@
         */
     int    closing;
 
+        /* flag: quit adbd when both ends close the
+        ** local service socket
+        */
+    int    exit_on_close;
+
         /* the asocket we are connected to
         */