advanced reboot: add soft reboot option

Adds a "Soft reboot" option which restarts zygote, as requested in JIRA
CYAN-3998

AOKP: This is based on the original commit. However we are handling the
      reboot dialog differently.

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