uncrypt: Communicate via /dev/socket/uncrypt.

We used to rely on files (e.g. /cache/recovery/command and
/cache/recovery/uncrypt_status) to communicate between uncrypt and its
caller (i.e. system_server). Since A/B devices may not have /cache
partitions anymore, we switch to socket communication instead.

We will keep the use of /cache/recovery/uncrypt_file to indicate the OTA
package to be uncrypt'd though. Because there is existing logic in
ShutdownThread.java that depends on the existence of the file to
detect pending uncrypt works. This part won't affect A/B devices without
/cache partitions, because such devices won't need uncrypt service (i.e
the real de-encrypt work) anyway.

Bug: 27176738
Change-Id: I481406e09e3ffc7b80f2c9e39003b9fca028742e