Add a skeleton of userspace reboot

This CL only draws boundaries between userspace and full reboots, and
adds some functionality that will be required for userspace reboot:

* Whenever device is shutting down is now controlled in reboot.cpp,
  since during userspace reboot this state can change.
* Now it's also possible to restart handling of control messages inside
  property service. In case of userspace reboot, init will restart it
  after stopping post-data services.
* New userspace-reboot-requested trigger is added similar to shutdown
  one for full reboot.

Test: adb reboot
Test: adb reboot userspace
Bug: 135984674
Change-Id: Id55a53ba781d2b90ce40449037b6d8d47e72c476