update shutdown sequence and use shutdown_timeout to cover all wait

- Use ro.build.shutdown_timeout to cover the total time for shutdown.
  Limit wait time for termination only to half of shutdown_timeout
  with max of 3 secs as process not terminating by that time
  will not terminate anyway. It is better to move to the next
  stage quickly. fsck time for user shutdown is excluded from timeout.
- Change last detach to kill, sync, and umount. Last detach did not
  work in many tests.
- add sync after emulated partitions umount as it can trigger
  change in /data.

bug: 36551393
Test: many reboots
Change-Id: Ib75dc19af79b8326b02ccef6b16a8817ae7f8b0e