manifest: android-8.1.0_r1 -> android-8.1.0_r4

Change-Id: I1b2f8331f1e3ca725495a2f3290e58d14b2ca69e
diff --git a/default.xml b/default.xml
index a47c45e..287cb7d 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_r1" />
+           revision="refs/tags/android-8.1.0_r4" />
 
   <default revision="refs/heads/staging/lineage-15.1"
            remote="github"