Switch external/icing/ from github to piper.

Suggested per http://g/android-new-project-reviewers/EY_i7nDUE88/C85jhLDEAQAJ.

Bug: 145010258
Change-Id: I18e92f677dcbc69198a50974a40691de8bd297b6
diff --git a/METADATA b/METADATA
index fc01d33..54818bf 100644
--- a/METADATA
+++ b/METADATA
@@ -6,13 +6,12 @@
 third_party {
   url {
     type: HOMEPAGE
-    value: "https://github.com/google/icing-search"
+    value: "http://go/icing-lib-notes"
   }
   url {
-    type: GIT
-    value: "https://github.com/google/icing-search.git"
+    type: PIPER
+    value: "http://google3/third_party/icing/"
   }
-  version: "6ac7095a212a01dd0d25e39843bb593486625370"
   last_upgrade_date { year: 2019 month: 10 day: 31 }
   license_type: NOTICE
 }