manifest: android-8.1.0_r15 -> android-8.1.0_r20

Change-Id: I90b75ab9f6c127152cbca802678c6d51970f7ce3
diff --git a/default.xml b/default.xml
index f3cf811..946368e 100644
--- a/default.xml
+++ b/default.xml
@@ -11,7 +11,7 @@
   <remote  name="aosp"
            fetch="https://android.googlesource.com"
            review="android-review.googlesource.com"
-           revision="refs/tags/android-8.1.0_r15" />
+           revision="refs/tags/android-8.1.0_r20" />
 
   <default revision="refs/heads/lineage-15.1"
            remote="github"