Make userspace_reboot.in_progress sysprop-as-api

sys.init.userspace_reboot.in_progress will be used to notify all
the processes (including vendor ones) that userspace reboot is
happening, hence it should be treated as stable public api.

All other sys.init.userspace_reboot.* props will be internal to /system
partition and don't require any stability guarantees.

Test: builds
Test: adb reboot userspace
Bug: 135984674
Change-Id: Ifb64a6bfae2de76bac67edea68df44e33c9cfe2d