Add SkipNativeNetworkCreation flag in NetworkAgentConfig

This flag allows ConnectivityService to skip native network creation,
deferring this responsibility to Tethering. This is necessary because:

- ConnectivityService requires the NetworkAgent to be in the CONNECTED
  state to configure routing and invokes NetworkCallback#onAvailable
  once ready.
- Tethering requires lower-layer interactions to be completed before
  notifying applications.

This approach ensures proper synchronization between ConnectivityService
and Tethering during network setup.

Long-term, all netd operations should be handled by ConnectivityService
when it supports early native network creation in the CONNECTING state.

As a mid-term solution, this flag enables Tethering to manage native
network creation within the IpServer.

Test: atest ConnectivityCoverageTests:android.net.connectivity.android.net.NetworkAgentConfigTest
Bug: 349487600
Change-Id: I84da9c74dc4d40c7cd36c6657f17a158affed689

tests/common/java/android/net/NetworkAgentConfigTest.kt

@@ -20,6 +20,7 @@ import androidx.test.filters.SmallTest
 import androidx.test.runner.AndroidJUnit4
 import com.android.modules.utils.build.SdkLevel.isAtLeastS
 import com.android.modules.utils.build.SdkLevel.isAtLeastT
+import com.android.modules.utils.build.SdkLevel.isAtLeastV
 import com.android.testutils.ConnectivityModuleTest
 import com.android.testutils.assertParcelingIsLossless
 import org.junit.Assert.assertEquals
@@ -47,6 +48,9 @@ class NetworkAgentConfigTest {
                 setLocalRoutesExcludedForVpn(true)
                 setVpnRequiresValidation(true)
             }
+            if (isAtLeastV()) {
+                setSkipNativeNetworkCreation(true)
+            }
         }.build()
         assertParcelingIsLossless(config)
     }
@@ -71,6 +75,9 @@ class NetworkAgentConfigTest {
                 setLocalRoutesExcludedForVpn(true)
                 setVpnRequiresValidation(true)
             }
+            if (isAtLeastV()) {
+                setSkipNativeNetworkCreation(true)
+            }
         }.build()
 
         assertTrue(config.isExplicitlySelected())
@@ -79,6 +86,9 @@ class NetworkAgentConfigTest {
         assertFalse(config.isPartialConnectivityAcceptable())
         assertTrue(config.isUnvalidatedConnectivityAcceptable())
         assertEquals("TEST_NETWORK", config.getLegacyTypeName())
+        if (isAtLeastV()) {
+            assertTrue(config.shouldSkipNativeNetworkCreation())
+        }
         if (isAtLeastT()) {
             assertTrue(config.areLocalRoutesExcludedForVpn())
             assertTrue(config.isVpnValidationRequired())