Move VpnDialogs into core.mk.

It is required by the APIs of user space VPN.
Also remove the dead VpnServices.

Bug: 5495078
Change-Id: I321b8da16898cd547ed79bea9c264b5e7bc781eb
diff --git a/target/product/core.mk b/target/product/core.mk
index ca5858f..d45ea17 100644
--- a/target/product/core.mk
+++ b/target/product/core.mk
@@ -43,7 +43,7 @@
     TelephonyProvider \
     TtsService \
     UserDictionaryProvider \
-    VpnServices \
+    VpnDialogs \
     apache-xml \
     bluetooth-health \
     bouncycastle \