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 7feab682)

Change-Id: I2e0ec81baa7b6cdb1ff0163c16f643c2549d74ab