add the pre-recovery service

Add a service called "pre-recovery" which is normally stopped but can
be started by the system server when we want to go into recovery.  It
will do any preparation needed (currently needed to handle update
packages that reside on an encrypted /data partition) and then set
sys.powerctl when it's ready to actually reboot.

Bug: 12188746
Change-Id: I894a4cb200395a0f6d7fe643ed4c2ac0a45d2052