Add a skeleton VpnManagerService, and start it on boot.

This adds a lot of unused code but this should make it easier to
review subsequent CLs.

Bug: 173331190
Test: builds, boots, "dumpsys vpnmanager" succeeds
Change-Id: Ied379654a0c3ab6242d3125661fe30f322395059

framework/src/android/net/VpnManager.java

@@ -76,6 +76,12 @@ public class VpnManager {
     @Deprecated
     public static final int TYPE_VPN_LEGACY = 3;
 
+    /**
+     * Channel for VPN notifications.
+     * @hide
+     */
+    public static final String NOTIFICATION_CHANNEL_VPN = "VPN";
+
     /** @hide */
     @IntDef(value = {TYPE_VPN_NONE, TYPE_VPN_SERVICE, TYPE_VPN_PLATFORM, TYPE_VPN_LEGACY})
     @Retention(RetentionPolicy.SOURCE)