Ensure MTU is set for new interfaces.

Setting the MTU for an interface should not be skipped when the
interface names are different. This occurs when a VPN network creates
a new interface with identical MTU.

Bug: 246398088
Test: atest ConnectivityServiceTest
Test: Manual test: Connect to VPN network and switch networks
Test: Confirm MTU of interface is set correctly with `adb shell ip addr`
Change-Id: I811a01feca2fb2130c57c6c924145314180434c5

tests/unit/java/com/android/server/ConnectivityServiceTest.java

@@ -17155,7 +17155,6 @@ public class ConnectivityServiceTest {
         mWiFiNetworkAgent.sendLinkProperties(new LinkProperties(lp2));
 
         waitForIdle();
-        // TODO(b/246398088): the MTU should be set on the new interface.
-        verify(mMockNetd, never()).interfaceSetMtu(eq(ifaceName2), eq(mtu));
+        verify(mMockNetd).interfaceSetMtu(eq(ifaceName2), eq(mtu));
     }
 }