userspace reboot: stop post-data services and wait for them to be killed

* Refactored code around stopping services a little bit to reuse it
  between full reboot and userspace reboot.
* Add a scope_guard to fallback to full reboot in case userspace reboot
  fails.
* In case of userspace reboot init will also wait for services to be
  terminated/killed and log the ones that didn't react to
  SIGTERM/SIGKILL in time.
* If some of the services didn't react to SIGKILL, fail userspace reboot.

Test: adb reboot userspace
Bug: 135984674
Change-Id: I820c7bc406169333b0f929f0eea028d8384eb2ac