Allow system server and uncrypt to operate pipe file

System server and uncrypt need to communicate with a named pipe on the
/cache partition. It will be created and deleted by system server.

Bug: 20012567
Bug: 20949086
(cherry picked from commit 70c6dbf0)
Change-Id: I4ddc523c2a0f4218877dae8f8a9b7fcf3f786625