Add a very basic userspace reboot watchdog

Watchdog is just a forked process that is going to fall back to the
full reboot in case device wasn't able to boot in given amount of time.

Currently this amount is hard-coded to 1 minute, but in the future it
will be controlled by a read-only property.

Also added sync calls before and after tearing down services.

Test: adb reboot userspace
Bug: 135984674
Change-Id: Ie6053c9446a6761deae6dc104036bb35b09ef0e2