Update to positive logic to better match the method naming

The method checkInterfacePresent seems that should return if the
interface is present or not, but it actually return an opposite
result. It may confuse the caller. E.g. The override method in
VpnTest assume it always returns true in the test. It will fail
the vpn start flow due to interface gone.

Bug: 169215213
Test: atest FrameworksNetTess
Change-Id: I3e9a64f0900336b8205674b94db68f090b4e9ff9

tests/net/java/com/android/server/connectivity/VpnTest.java

@@ -1263,7 +1263,7 @@ public class VpnTest {
         }
 
         @Override
-        public boolean checkInterfacePresent(final Vpn vpn, final String iface) {
+        public boolean isInterfacePresent(final Vpn vpn, final String iface) {
             return true;
         }
     }