Clear IPv4 address on tethering teardown.

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net passes
    - wlan0 in STA mode, wlan1 up/down in AP mode
      no lingering IPv4 mode
    - USB tethering up/down works
    - bluetooth tethering yields:
05-18 17:50:49.726   719   756 D TetherController: untetherInterface(bt-pan)
05-18 17:50:49.729  1194  1230 E Tethering: [bt-pan] ERROR Failed to clear IPv4 address on interface bt-pan: java.lang.IllegalStateException: command '224 interface setcfg bt-pan 0.0.0.0 0' failed with '400 224 Failed to clear address (No such device)'
      which is acceptable (no actual crash, just a log message)
Bug: 79905644
Merged-In: Ie898adc4efbb7376f0297abacdfe39c8700f0722
Merged-In: I9eb44eaf4e99fa85fff2909524ee88673bdcf1fd
Merged-In: Iaf29788a6692d810f3160e3f21d06b7452ecbaa6

(cherry picked from commit 472276a874)

Change-Id: Icb5c4f7971af4715c7662f80194b4c1ce369a135

services/core/java/com/android/server/connectivity/tethering/TetherInterfaceStateMachine.java

@@ -185,7 +185,12 @@ public class TetherInterfaceStateMachine extends StateMachine {
 
     private boolean startIPv4() { return configureIPv4(true); }
 
-    private void stopIPv4() { configureIPv4(false); }
+    private void stopIPv4() {
+        configureIPv4(false);
+        // NOTE: All of configureIPv4() will be refactored out of existence
+        // into calls to InterfaceController, shared with startIPv4().
+        mInterfaceCtrl.clearIPv4Address();
+    }
 
     // TODO: Refactor this in terms of calls to InterfaceController.
     private boolean configureIPv4(boolean enabled) {

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

@@ -779,11 +779,12 @@ public class TetheringTest {
         sendWifiApStateChanged(WIFI_AP_STATE_ENABLED, TEST_WLAN_IFNAME, IFACE_IP_MODE_TETHERED);
         mLooper.dispatchAll();
 
-        // We verify get/set called twice here: once for setup and once during
+        // We verify get/set called thrice here: once for setup and twice during
         // teardown because all events happen over the course of the single
-        // dispatchAll() above.
+        // dispatchAll() above. Note that once the TISM IPv4 address config
+        // code is refactored the two calls during shutdown will revert to one.
         verify(mNMService, times(2)).getInterfaceConfig(TEST_WLAN_IFNAME);
-        verify(mNMService, times(2))
+        verify(mNMService, times(3))
                 .setInterfaceConfig(eq(TEST_WLAN_IFNAME), any(InterfaceConfiguration.class));
         verify(mNMService, times(1)).tetherInterface(TEST_WLAN_IFNAME);
         verify(mWifiManager).updateInterfaceIpState(

tests/net/java/com/android/server/connectivity/tethering/TetherInterfaceStateMachineTest.java

@@ -173,6 +173,7 @@ public class TetherInterfaceStateMachineTest {
         dispatchCommand(TetherInterfaceStateMachine.CMD_TETHER_UNREQUESTED);
         InOrder inOrder = inOrder(mNMService, mStatsService, mTetherHelper);
         inOrder.verify(mNMService).untetherInterface(IFACE_NAME);
+        inOrder.verify(mNMService).setInterfaceConfig(eq(IFACE_NAME), any());
         inOrder.verify(mTetherHelper).updateInterfaceState(
                 mTestedSm, STATE_AVAILABLE, TETHER_ERROR_NO_ERROR);
         inOrder.verify(mTetherHelper).updateLinkProperties(
@@ -270,6 +271,7 @@ public class TetherInterfaceStateMachineTest {
         inOrder.verify(mNMService).stopInterfaceForwarding(IFACE_NAME, UPSTREAM_IFACE);
         inOrder.verify(mNMService).disableNat(IFACE_NAME, UPSTREAM_IFACE);
         inOrder.verify(mNMService).untetherInterface(IFACE_NAME);
+        inOrder.verify(mNMService).setInterfaceConfig(eq(IFACE_NAME), any());
         inOrder.verify(mTetherHelper).updateInterfaceState(
                 mTestedSm, STATE_AVAILABLE, TETHER_ERROR_NO_ERROR);
         inOrder.verify(mTetherHelper).updateLinkProperties(