Backend: hide shcon workaround stderr on (likely) working recoveries

- some recoveries don't allow reading/changing shell context and will show an error:

sh: write error: Invalid argument

- these recoveries seem to allow loop mounts unlike most LOS Recovery builds lately, so hide the error output

META-INF/com/google/android/update-binary

@@ -58,7 +58,7 @@ mount_apex() {
   setup_mountpoint /apex;
   mount -t tmpfs tmpfs /apex -o mode=755 && touch /apex/apextmp;
   shcon=$(cat /proc/self/attr/current);
-  echo "u:r:su:s0" > /proc/self/attr/current; # work around LOS Recovery not allowing loop mounts in recovery context
+  echo "u:r:su:s0" > /proc/self/attr/current 2>/dev/null; # work around LOS Recovery not allowing loop mounts in recovery context
   minorx=1;
   [ -e /dev/block/loop1 ] && minorx=$(ls -l /dev/block/loop1 | awk '{ print $6 }');
   num=0;
@@ -94,7 +94,7 @@ mount_apex() {
       *) mount -o bind $apex $dest && echo "$dest (bind)" >&2;;
     esac;
   done;
-  echo "$shcon" > /proc/self/attr/current;
+  echo "$shcon" > /proc/self/attr/current 2>/dev/null;
   for var in $(grep -o 'export .* /.*' /system_root/init.environ.rc | awk '{ print $2 }'); do
     eval OLD_${var}=\$$var;
   done;