Set updatable:false explicity.

This is currently the default value but this is changing.

Bug: 180375550
Test: Treehugger
Change-Id: Ibf0523c03c3559f78f8e9f191e0487237ae44564
diff --git a/apex/appsearch/Android.bp b/apex/appsearch/Android.bp
index b014fdc..ab44dd9 100644
--- a/apex/appsearch/Android.bp
+++ b/apex/appsearch/Android.bp
@@ -21,6 +21,7 @@
     ],
     key: "com.android.appsearch.key",
     certificate: ":com.android.appsearch.certificate",
+    updatable: false,
 }
 
 apex_key {