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

(cherry picked from commit 7feab6823885cb591f9239e01235d2ed529f2b69)

Change-Id: I2e0ec81baa7b6cdb1ff0163c16f643c2549d74ab
diff --git a/init/reboot.cpp b/init/reboot.cpp
index 7970b4a..c1edf7c 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;