installer/addon: Check A/B partitioning with ro.build.ab_update instead

* As osm0sis suggested [here](opengapps#879 (comment))
  some A only devices, seem to report slot as _a.

* So check with ro.build.ab_update instead

scripts/bkup_tail.sh

@@ -2,9 +2,9 @@ EOF
 }
 
 mount_generic() {
-  local active_slot=$(getprop ro.boot.slot_suffix)
+  local device_abpartition=$(getprop ro.build.ab_update)
   local partitions="$*"
-  if [ -z "$active_slot" ]; then
+  if [ -z "$device_abpartition" ]; then
     # We're on an A only device
     local partition
     for partition in $partitions; do

scripts/templates/installer.sh

@@ -1067,10 +1067,7 @@ ui_print " "
 device_abpartition=false
 system_as_root=`getprop ro.build.system_root_image`
 if [ "$system_as_root" = "true" ]; then
-  active_slot=`getprop ro.boot.slot_suffix`
-  if [ ! -z "$active_slot" ]; then
-    device_abpartition=true
-  fi
+    device_abpartition=$(getprop ro.build.ab_update)
 fi
 
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