set zero shutdown timeout for eng build

- still it will take time to kill services, < 3 secs in tested device.

bug: 36678028
Test: reboot
Change-Id: I3f3eb83aede8cd950da12e3fcc259eeaf8517c3b
diff --git a/init/reboot.cpp b/init/reboot.cpp
index 2844d78..e34abdb 100644
--- a/init/reboot.cpp
+++ b/init/reboot.cpp
@@ -341,7 +341,14 @@
     }
 
     /* TODO update default waiting time based on usage data */
-    unsigned int shutdownTimeout = android::base::GetUintProperty("ro.build.shutdown_timeout", 10u);
+    constexpr unsigned int shutdownTimeoutDefault = 10;
+    unsigned int shutdownTimeout = shutdownTimeoutDefault;
+    if (SHUTDOWN_ZERO_TIMEOUT) {  // eng build
+        shutdownTimeout = 0;
+    } else {
+        shutdownTimeout =
+            android::base::GetUintProperty("ro.build.shutdown_timeout", shutdownTimeoutDefault);
+    }
     LOG(INFO) << "Shutdown timeout: " << shutdownTimeout;
 
     static const constexpr char* shutdown_critical_services[] = {"vold", "watchdogd"};