Add /dev/socket/uncrypt.

system_server used to communicate with uncrypt via files (e.g.
/cache/recovery/command and /cache/recovery/uncrypt_status). Since A/B
devices may not have /cache partitions anymore, we switch to communicate
via /dev/socket/uncrypt to allow things like factory reset to keep
working.

Bug: 27176738
Change-Id: I73b6d6f1ecdf16fd4f3600b5e524da06f35b5bca