allow system_server to write to cameraserver's /proc/<pid>/timerslack_ns

This is needed for timerslack functionality which should be present in
most kernels going forward

Test: system_server can write to cameraserver files
Change-Id: I85797128b1467d92eb354364de8eb60f8e45c931

private/system_server.te

@@ -107,6 +107,9 @@ 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 cameraserver:file w_file_perms;
+
 # Read /proc/pid data for all domains. This is used by ProcessCpuTracker
 # within system_server to keep track of memory and CPU usage for
 # all processes on the device. In addition, /proc/pid files access is needed