Backend: fix loop mounts on newer Lineage-based recoveries

- loops would fail only in LOS Recovery from direct mount .img and losetup .img+loop mount with, e.g. "mount: mounting /dev/block/loop0 on /apex/com.android.adbd failed: Invalid argument"
- this started ~LOS19, I believe, though I haven't tracked down the specific change yet
- confusing to diagnose, since it always works in recovery adb shell, until I realized there was a context difference: "su" for adb shell and "recovery" for zip flashes
- switch shell context temporarily to "su" to do apex mounts, then return to original context (currently only necessary for mounting loops)

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

@@ -54,9 +54,11 @@ setup_mountpoint() {
 is_mounted() { mount | grep -q " $1 "; }
 mount_apex() {
   [ -d /system_root/system/apex ] || return 1;
-  local apex dest loop minorx num var;
+  local apex dest loop minorx num shcon var;
   setup_mountpoint /apex;
   mount -t tmpfs tmpfs /apex -o mode=755 && touch /apex/apextmp;
+  shcon=$(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
   minorx=1;
   [ -e /dev/block/loop1 ] && minorx=$(ls -l /dev/block/loop1 | awk '{ print $6 }');
   num=0;
@@ -92,6 +94,7 @@ mount_apex() {
       *) mount -o bind $apex $dest && echo "$dest (bind)" >&2;;
     esac;
   done;
+  echo "$shcon" > /proc/self/attr/current;
   for var in $(grep -o 'export .* /.*' /system_root/init.environ.rc | awk '{ print $2 }'); do
     eval OLD_${var}=\$$var;
   done;

META-INF/com/google/android/updater-script

@@ -3,4 +3,4 @@
 # Dummy file; update-binary is a shell script (DO NOT CHANGE)
 #
 #
-# AK_BASE_VERSION=20230525
+# AK_BASE_VERSION=20230526