advanced reboot: add soft reboot option

Adds a "Soft reboot" option which restarts zygote.

Also includes "ShutdownThread: set soft reboot trigger accordingly"
commit by Dan Pasanen <dan.pasanen@gmail.com>

Signed-off-by: Roman Birg <roman@cyngn.com>
Change-Id: Ia6e5ed9b053e1c5ebd1fc55e5692858b97f7e0fc