manifest: update to android-9.0.0_r31

Change-Id: I0f49c77f61c63f2af20154d124d816e940d411e4

default.xml

@@ -4,7 +4,7 @@
   <remote  name="aosp"
            fetch="https://android.googlesource.com"
            review="https://android-review.googlesource.com/" />
-  <default revision="refs/tags/android-9.0.0_r30"
+  <default revision="refs/tags/android-9.0.0_r31"
            remote="aosp"
            sync-c="true"
            sync-j="12" />