android: update revision to 8.1.0_r20

Change-Id: I5961eea3322e3827bc4b0a30077f82bfbfaa5e57
diff --git a/default.xml b/default.xml
index 4e048b6..6ab27ce 100644
--- a/default.xml
+++ b/default.xml
@@ -3,7 +3,7 @@
   <remote  name="aosp"
            fetch="https://android.googlesource.com"
            review="https://android-review.googlesource.com/" />
-  <default revision="refs/tags/android-8.1.0_r15"
+  <default revision="refs/tags/android-8.1.0_r20"
            remote="aosp"
            sync-c="true"
            sync-j="12" />