Remove the usage of hidden API - setIsAvailable() from Vpn

In fact, NetworkAgent#getLegacyNetworkInfo() will call this API,
so VPN doesn't need to call it.
Remove the redundant behavior and the usage of hidden API.

Bug: 182963397
Test: m
Change-Id: I3b88f22fffde3dd4000d28342ed07209c7121a37

services/core/java/com/android/server/connectivity/Vpn.java

@@ -1288,7 +1288,6 @@ public class Vpn {
         });
         mNetworkAgent.setUnderlyingNetworks((mConfig.underlyingNetworks != null)
                 ? Arrays.asList(mConfig.underlyingNetworks) : null);
-        mNetworkInfo.setIsAvailable(true);
         updateState(DetailedState.CONNECTED, "agentConnect");
     }