shill: increase kill timeout to 10s

This CL increases the kill timeout for the shill job to 10s and makes it
stop on 'starting pre-shutdown' in order to ensure that shill has enough
time to complete its termination clearly. It also adds the bootstat
'shill-start' and 'shill-stop' for tracking the startup and shutdown
time of shill.

BUG=chromium:303395
CQ-DEPEND=CL:171657
TEST=Tested the following:
1. Connect to WiFi and cellular, and then run `stop shill` to terminate
   shill. Verify that shill terminates properly.
2. Connect to WiFi and cellular, and then run `restart shill` to restart
   shill. Verify that shill terminates and restarts properly.
3. Modify shill to artificially make it take more than 10s to terminate.
   - Run `stop shill` and verify that upstart kills shill after the 10s
     wait times out.
   - Run `init 6` and verify that chromeos_shutdown is not started
     before shill is terminated, i.e. bootstat_summary shows that
     'shill-stop' happens before 'shutdown-start'.

Change-Id: I2f7dc742f99b7c818af5c26dccd8e9952be57a97
Reviewed-on: https://chromium-review.googlesource.com/171668
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>