Auto-call prepare() for new always-on VPNs

Existing VPN apps don't tend to call prepare() inside their VpnService
(they usually do that much earlier) so this eases the migration path
quite a bit and gives the networking system an advance cue as to which
app is about to take over.

Bug: 26891808
Change-Id: Ibda40d11a7e6fa3e764c18fcc4d502c1a4959f9b
diff --git a/services/core/java/com/android/server/connectivity/Vpn.java b/services/core/java/com/android/server/connectivity/Vpn.java
index fd9abff..f231f92 100644
--- a/services/core/java/com/android/server/connectivity/Vpn.java
+++ b/services/core/java/com/android/server/connectivity/Vpn.java
@@ -190,6 +190,7 @@
             if (!setPackageAuthorization(packageName, true)) {
                 return false;
             }
+            prepareInternal(packageName);
         }
 
         // Save the new package name in Settings.Secure.