Add selinux rules for additional file contexts in userdebug

These rules allow the additional tracepoints we need for running traceur
in userdebug builds to be writeable.

Bug: 37110010
Test: I'm testing by running atrace -l and confirming that the
tracepoints that I'm attempting to enable are available.

Change-Id: Ia352100ed67819ae5acca2aad803fa392d8b80fd
diff --git a/private/shell.te b/private/shell.te
index c24bfd3..c0b4ee5 100644
--- a/private/shell.te
+++ b/private/shell.te
@@ -7,6 +7,10 @@
 allow shell debugfs_trace_marker:file getattr;
 allow shell atrace_exec:file rx_file_perms;
 
+userdebug_or_eng(`
+  allow shell tracing_shell_writable_debug:file rw_file_perms;
+')
+
 # Run app_process.
 # XXX Transition into its own domain?
 app_domain(shell)