android update to r31

Change-Id: Ie261f4bb082fabf63d1b9956571c058dd772a5f6

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-8.0.0_r17"
+  <default revision="refs/tags/android-8.0.0_r31"
            remote="aosp"
            sync-c="true"
            sync-j="4" />