set default shutdown timeout to 6 secs

- Test data shows that most shutdown finishes in 6 secs.
- The original 10 secs is too long wih no shutdown animation
  running in screen.

bug: 36657139
Test: check time with reboot
Change-Id: I9a805ddfde8156b066485902048d0cd01365c453
diff --git a/init/reboot.cpp b/init/reboot.cpp
index 395d21b..172fc77 100644
--- a/init/reboot.cpp
+++ b/init/reboot.cpp
@@ -318,8 +318,7 @@
         abort();
     }
 
-    /* TODO update default waiting time based on usage data */
-    constexpr unsigned int shutdownTimeoutDefault = 10;
+    constexpr unsigned int shutdownTimeoutDefault = 6;
     unsigned int shutdownTimeout = shutdownTimeoutDefault;
     if (SHUTDOWN_ZERO_TIMEOUT) {  // eng build
         shutdownTimeout = 0;