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
diff --git a/init/reboot.h b/init/reboot.h
index 07dcb6e..81c3edc 100644
--- a/init/reboot.h
+++ b/init/reboot.h
@@ -23,8 +23,9 @@
 namespace init {
 
 // Parses and handles a setprop sys.powerctl message.
-bool HandlePowerctlMessage(const std::string& command);
+void HandlePowerctlMessage(const std::string& command);
 
+bool IsShuttingDown();
 }  // namespace init
 }  // namespace android