init: don't sync() before shutting down services

Devices in the lab are hitting an issue where they're getting stuck
likely in the sync() call in DoReboot() before we start the reboot
monitor thread and before we shut down services.

It's possible that concurrent writing to RW file systems is causing
this sync() call to take essentially forever.  To protect against
this, we need to remove this sync().  Note that we will still call
sync() after shutting down services.

Note that the service shutdown code has a timeout and there is a
reboot monitor thread that will shutdown the device if more than 30
seconds pass above that timeout.  This change increases that timeout
to 300 seconds to give the final sync() calls explicitly more time to
finish.

Bug: 150863651
Test: reboot functions normally
Test: put an infinite loop in DoReboot and the the reboot monitor thread
      triggers and shuts down the device appropriately
Merged-In: I6fd7d3a25d3225081388e39a14c9fdab21b592ba
Change-Id: I6fd7d3a25d3225081388e39a14c9fdab21b592ba
(cherry picked from commit 10615eb3)