VPN: convert prepare() into a form which helps solve race conditions.

When someone tries to revoke packageA, it is possible that packageA is
already revoked by packageB. In this case packageB should not be revoked,
and the new prepare() can help solve this problem.

Change-Id: Iee056a191dd99467b8ad1b5379a17b02d404bad1

services/java/com/android/server/ConnectivityService.java

@@ -2485,8 +2485,8 @@ public class ConnectivityService extends IConnectivityManager.Stub {
      * @hide
      */
     @Override
-    public String prepareVpn(String packageName) {
-        return mVpn.prepare(packageName);
+    public boolean prepareVpn(String oldPackage, String newPackage) {
+        return mVpn.prepare(oldPackage, newPackage);
     }
 
     /**