Stop listening for WiFi interface status changes

Bug: 29931437
Test: Can toggle tethering on and off repeated on shamu and an
      upcoming device, both devices without a SIM.
Test: Can toggle tethering on and off repeated on an
      upcoming device with a T-Mobile SIM.

Change-Id: I4986b1a5f6ae9b45010bd30639806f7ed63a605c
(cherry picked from commit d30aaeb2148edbc0c7a97699a41c450c0591b983)
diff --git a/services/core/java/com/android/server/connectivity/Tethering.java b/services/core/java/com/android/server/connectivity/Tethering.java
index bef48d6..6f67b6f 100644
--- a/services/core/java/com/android/server/connectivity/Tethering.java
+++ b/services/core/java/com/android/server/connectivity/Tethering.java
@@ -270,14 +270,16 @@
                     trackNewTetherableInterface(iface, interfaceType);
                 }
             } else {
-                if (interfaceType == ConnectivityManager.TETHERING_USB) {
-                    // ignore usb0 down after enabling RNDIS
-                    // we will handle disconnect in interfaceRemoved instead
-                    if (VDBG) Log.d(TAG, "ignore interface down for " + iface);
-                } else if (tetherState != null) {
+                if (interfaceType == ConnectivityManager.TETHERING_BLUETOOTH) {
                     tetherState.mStateMachine.sendMessage(
                             TetherInterfaceStateMachine.CMD_INTERFACE_DOWN);
                     mTetherStates.remove(iface);
+                } else {
+                    // Ignore usb0 down after enabling RNDIS.
+                    // We will handle disconnect in interfaceRemoved.
+                    // Similarly, ignore interface down for WiFi.  We monitor WiFi AP status
+                    // through the WifiManager.WIFI_AP_STATE_CHANGED_ACTION intent.
+                    if (VDBG) Log.d(TAG, "ignore interface down for " + iface);
                 }
             }
         }