Explicitly set apex updatable to false.

This is currently the default, but the default is changing to true so
this change is necessary to keep the build working.

Test: Treehugger
Bug: 180375550
Change-Id: I5b685ea9169177b356276f301c29065c20e9bdee
diff --git a/apex/Android.bp b/apex/Android.bp
index a7137d9..25a4909 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -9,6 +9,8 @@
 
     key: "com.android.telephony.key",
     certificate: ":com.android.telephony.certificate",
+
+    updatable: false,
 }
 
 apex {