The only file in /proc/<pid>/ that is world writeable is
/proc/<pid>/timerslack_ns so granting selinux write permission to
a process's /proc/<pid>/ file only allows writing to timerslack_ns
(unless the process is running as system UID).

Addresses denials such as:
avc: denied { write } for comm="Binder:1117_2" name="timerslack_ns"
dev="proc" ino=27908 scontext=u:r:system_server:s0
tcontext=u:r:priv_app:s0 tclass=file permissive=1

Bug: 30675296
Test: build
Change-Id: I2cee8ce73a0dc05d771881c36da2fde5411859fb