system_server: allow writing to timerslack_ns

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

private/system_server.te

@@ -106,7 +106,9 @@ allow system_server hal_camera:process { getsched setsched };
 allow system_server mediaserver:process { getsched setsched };
 allow system_server bootanim:process { getsched setsched };
 
-# Allow system_server to write to cameraserver's /proc/<pid>/timerslack_ns
+# Allow system_server to write to /proc/<pid>/timerslack_ns
+allow system_server appdomain:file w_file_perms;
+allow system_server audioserver:file w_file_perms;
 allow system_server cameraserver:file w_file_perms;
 
 # Read /proc/pid data for all domains. This is used by ProcessCpuTracker