Log UserspaceRebootReported atom from system_server

UserspaceRebootLogger class is introduced to encapsulate (tiny) state
machine for logging of userspace reboot.

So far state is stored in a persistent property, but that can be changed
in the future.

Unit tests will be added in a follow-up CL.

Test: adb shell svc power reboot userspace
Bug: 148767783
Change-Id: Ib9e2c6cef7094ccac7862be249d619212f0013fd
(cherry picked from commit d5594e2f901ba7eced97b35da8cf0f0a3f6242bd)