system_server: access to /proc/sys/fs/pipe-max-size

Label /proc/sys/fs/pipe-max-size with new type proc_pipe_conf and give
system_server access to it.

Addresses this denial:
avc: denied { read } for name="pipe-max-size" dev="proc" ino=93817
scontext=u:r:system_server:s0 tcontext=u:object_r:proc:s0 tclass=file
permissive=0

Bug: 69175449
Bug: 69324398
Test: sailfish boots
Test: adb bugreport
Test: craft an unresponsive app, trigger ANR, make sure traces are dumped
into /data/anr
Above denial from system_server not observed, no denials to proc_pipe_conf
observed.
Change-Id: I7c71f05820a4945ba982e29f76e9d9f4458b2b59