Add API for apps to check if they are the network owner

This CL extends NetworkCapabilities#mEstablishingVpnAppUid
to the network owner app UID and introduces a new public API
to get this owner app's UID.

Bug: 142072839
Test: atest FrameworksNetTests
Change-Id: Id83cdea62b89b586aff74e51e3fee60e53d37d4c

tests/net/common/java/android/net/NetworkCapabilitiesTest.java

@@ -269,6 +269,7 @@ public class NetworkCapabilitiesTest {
             .setUids(uids)
             .addCapability(NET_CAPABILITY_EIMS)
             .addCapability(NET_CAPABILITY_NOT_METERED);
+        netCap.setOwnerUid(123);
         assertParcelingIsLossless(netCap);
         netCap.setSSID(TEST_SSID);
         assertParcelSane(netCap, 13);