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
diff --git a/core/java/android/net/IConnectivityManager.aidl b/core/java/android/net/IConnectivityManager.aidl
index e3c49ac..57f5967 100644
--- a/core/java/android/net/IConnectivityManager.aidl
+++ b/core/java/android/net/IConnectivityManager.aidl
@@ -101,7 +101,7 @@
 
     void protectVpn(in ParcelFileDescriptor socket);
 
-    String prepareVpn(String packageName);
+    boolean prepareVpn(String oldPackage, String newPackage);
 
     ParcelFileDescriptor establishVpn(in VpnConfig config);