Add sysprops for start & end of userspace reboot

There will be useful in debugging/logging events to statsd.

Also as part of this CL, sys.init.userspace_reboot.in_progress property
is now used as a mean of synchronization. It is set directly in
DoUserspaceReboot, to make sure that all the setprop actions triggered
by userspace-reboot-requested were processed.

Test: adb reboot userspace
Test: adb shell getprop sys.init.userspace_reboot.last_started
Test: adb shell getprop sys.init.userspace_reboot.last_finished
Bug: 135984674
Change-Id: I9debcd4f058e790855200d5295344dafb30e496a