Send tethered interface callbacks based on interface existence.

Currently, callbacks for tethered interface availability are sent
based on interface link state. This is incorrect:

- It is acceptable and expected to be able to enable tethering on
  an interface that does not yet have link up.
- It doesn't seem useful to disable Ethernet tethering if link
  temporarily goes down.

Instead, only base the callbacks on whether an interface exists
or not.

Bug: 150644681
Test: WIP tests in aosp/1260100 pass
Change-Id: I015219cbd03626c38fb5156d0cb2991ba7d7def7

service-t/src/com/android/server/ethernet/EthernetTracker.java

@@ -262,6 +262,7 @@ final class EthernetTracker {
 
     private void removeInterface(String iface) {
         mFactory.removeInterface(iface);
+        maybeUpdateServerModeInterfaceState(iface, false);
     }
 
     private void stopTrackingInterface(String iface) {
@@ -308,6 +309,8 @@ final class EthernetTracker {
 
             Log.d(TAG, "Started tracking interface " + iface);
             mFactory.addInterface(iface, hwAddress, nc, ipConfiguration);
+        } else {
+            maybeUpdateServerModeInterfaceState(iface, true);
         }
 
         // Note: if the interface already has link (e.g., if we crashed and got
@@ -341,12 +344,9 @@ final class EthernetTracker {
             }
             mListeners.finishBroadcast();
         }
-
-        updateServerModeInterfaceState(iface, up, mode);
     }
 
-    private void updateServerModeInterfaceState(String iface, boolean up, int mode) {
-        final boolean available = up && (mode == INTERFACE_MODE_SERVER);
+    private void maybeUpdateServerModeInterfaceState(String iface, boolean available) {
         if (available == mTetheredInterfaceWasAvailable || !iface.equals(mDefaultInterface)) return;
 
         final int pendingCbs = mTetheredInterfaceRequests.beginBroadcast();