remount: Allow 'shell' to run 'remount_exec' domain

The domain of 'remount' used to be 'system_file', which is
read-executable by 'shell'. However when I submitted aosp/1878144, the
domain of 'remount' became 'remount_exec', and I forgot to allow
'shell' to read-execute the new 'remount_exec' domain.
This makes `adb remount` w/o root to produce sub-par error message:
  $ adb remount [-h]
  /system/bin/sh: remount: inaccessible or not found

Allow 'shell' to read-execute 'remount_exec', so that the user can get a
proper error message when not running as root, and help (-h) message can
be displayed:
  $ adb remount
  Not running as root. Try "adb root" first.
  $ adb remount -h
  Usage: remount ...

Bug: 241688845
Test: adb unroot && adb remount [-h]
Change-Id: I5c105eaffa7abddaf14a9d0120fd6b71749c7977
diff --git a/private/shell.te b/private/shell.te
index c20e612..6a7c629 100644
--- a/private/shell.te
+++ b/private/shell.te
@@ -121,6 +121,9 @@
   allow shell profcollectd:binder call;
 ')
 
+# Allow shell to run remount command.
+allow shell remount_exec:file rx_file_perms;
+
 # Allow shell to call perf_event_open for profiling other shell processes, but
 # not the whole system.
 allow shell self:perf_event { open read write kernel };