Improve ConnectivityManager docs

Also fix some permission problems.
bug:5738328

Change-Id: Ib32c223f425b1fc03b8cce528456bcb50b540fdf
diff --git a/services/java/com/android/server/connectivity/Vpn.java b/services/java/com/android/server/connectivity/Vpn.java
index bb7334a..533db46 100644
--- a/services/java/com/android/server/connectivity/Vpn.java
+++ b/services/java/com/android/server/connectivity/Vpn.java
@@ -462,6 +462,7 @@
      * secondary thread to perform connection work, returning quickly.
      */
     public void startLegacyVpn(VpnProfile profile, KeyStore keyStore, LinkProperties egress) {
+        enforceControlPermission();
         if (!keyStore.isUnlocked()) {
             throw new IllegalStateException("KeyStore isn't unlocked");
         }