Change pre-recovery into two services

/system/bin/uncrypt needs to be triggered to prepare the OTA package
before rebooting into the recovery. Separate pre-recovery (uncrypt)
into two services: uncrypt that does the uncryption work and
pre-recovery that actually reboots the device into recovery.

Also create /cache/recovery on post-fs in case it doesn't exist.

Bug: 20012567
Bug: 20949086
Change-Id: If67fe1e9ee6279593d2788452febcd3f0fe714c2