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
(cherry picked from commit e48aed0f)
Change-Id: I9877cd6ac9412ea6a566bb1ec0807940c7a38ce5