Allow shell /proc/loadavg access

Needed for the bionic stdlib.getloadavg test.

Access to /proc/loadavg was inadvertantly removed when a new label was
assigned to that file in system/sepolicy commit
8c2323d3f9d052ddb7c2a3ef87db59d13f5021c0.

Addresses the following denial:

  CtsBionicTestCa: type=1400 audit(0.0:188192): avc: denied { read } for name="loadavg" dev="proc" ino=4026531959 scontext=u:r:shell:s0 tcontext=u:object_r:proc_loadavg:s0 tclass=file permissive=0

Bug: 124024827
Test: compiles
Change-Id: Iadb5c98cb96f69ddc9418a64720370adae1bb51f
diff --git a/public/shell.te b/public/shell.te
index 7201df0..dc53049 100644
--- a/public/shell.te
+++ b/public/shell.te
@@ -131,6 +131,7 @@
   proc_asound
   proc_filesystems
   proc_interrupts
+  proc_loadavg # b/124024827
   proc_meminfo
   proc_modules
   proc_pid_max