Refactor "avoid bad wifi" logic into a utility class

Additionally, add this utility class to IpManager for compatibility
verification.  A follow-on CL will make use of IpManager's local
AvoidBadWifiTracker.

Bug: 31827713
Change-Id: If8c56c3f8076d6a5157ea180e361bbdadc2bc1dd

services/net/java/android/net/util/AvoidBadWifiTracker.java

@@ -0,0 +1,139 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package android.net.util;
+
+import android.content.BroadcastReceiver;
+import android.content.ContentResolver;
+import android.content.Context;
+import android.content.Intent;
+import android.content.IntentFilter;
+import android.database.ContentObserver;
+import android.net.Uri;
+import android.os.Handler;
+import android.os.Message;
+import android.os.UserHandle;
+import android.provider.Settings;
+import android.util.Slog;
+
+import com.android.internal.annotations.VisibleForTesting;
+import com.android.internal.R;
+
+import static android.provider.Settings.Global.NETWORK_AVOID_BAD_WIFI;
+
+/**
+ * A class to encapsulate management of the "Smart Networking" capability of
+ * avoiding bad Wi-Fi when, for example upstream connectivity is lost or
+ * certain critical link failures occur.
+ *
+ * This enables the device to switch to another form of connectivity, like
+ * mobile, if it's available and working.
+ *
+ * The Runnable |cb|, if given, is called on the supplied Handler's thread
+ * whether the computed "avoid bad wifi" value changes.
+ *
+ * Disabling this reverts the device to a level of networking sophistication
+ * circa 2012-13 by disabling disparate code paths each of which contribute to
+ * maintaining continuous, working Internet connectivity.
+ *
+ * @hide
+ */
+public class AvoidBadWifiTracker {
+    private static String TAG = AvoidBadWifiTracker.class.getSimpleName();
+
+    private final Context mContext;
+    private final Handler mHandler;
+    private final Runnable mReevaluateRunnable;
+    private final SettingObserver mSettingObserver;
+    private volatile boolean mAvoidBadWifi = true;
+
+    public AvoidBadWifiTracker(Context ctx, Handler handler) {
+        this(ctx, handler, null);
+    }
+
+    public AvoidBadWifiTracker(Context ctx, Handler handler, Runnable cb) {
+        mContext = ctx;
+        mHandler = handler;
+        mReevaluateRunnable = () -> { if (update() && cb != null) cb.run(); };
+        mSettingObserver = new SettingObserver();
+
+        final IntentFilter intentFilter = new IntentFilter();
+        intentFilter.addAction(Intent.ACTION_CONFIGURATION_CHANGED);
+        mContext.registerReceiverAsUser(new BroadcastReceiver() {
+            public void onReceive(Context context, Intent intent) {
+                reevaluate();
+            }
+        }, UserHandle.ALL, intentFilter, null, null);
+
+        update();
+    }
+
+    public boolean currentValue() {
+        return mAvoidBadWifi;
+    }
+
+    /**
+     * Whether the device or carrier configuration disables avoiding bad wifi by default.
+     */
+    public boolean configRestrictsAvoidBadWifi() {
+        return (mContext.getResources().getInteger(R.integer.config_networkAvoidBadWifi) == 0);
+    }
+
+    /**
+     * Whether we should display a notification when wifi becomes unvalidated.
+     */
+    public boolean shouldNotifyWifiUnvalidated() {
+        return configRestrictsAvoidBadWifi() && getSettingsValue() == null;
+    }
+
+    public String getSettingsValue() {
+        final ContentResolver resolver = mContext.getContentResolver();
+        return Settings.Global.getString(resolver, NETWORK_AVOID_BAD_WIFI);
+    }
+
+    @VisibleForTesting
+    public void reevaluate() {
+        mHandler.post(mReevaluateRunnable);
+    }
+
+    public boolean update() {
+        final boolean settingAvoidBadWifi = "1".equals(getSettingsValue());
+        final boolean prev = mAvoidBadWifi;
+        mAvoidBadWifi = settingAvoidBadWifi || !configRestrictsAvoidBadWifi();
+        return mAvoidBadWifi != prev;
+    }
+
+    private class SettingObserver extends ContentObserver {
+        private final Uri mUri = Settings.Global.getUriFor(NETWORK_AVOID_BAD_WIFI);
+
+        public SettingObserver() {
+            super(null);
+            final ContentResolver resolver = mContext.getContentResolver();
+            resolver.registerContentObserver(mUri, false, this);
+        }
+
+        @Override
+        public void onChange(boolean selfChange) {
+            Slog.wtf(TAG, "Should never be reached.");
+        }
+
+        @Override
+        public void onChange(boolean selfChange, Uri uri) {
+            if (!mUri.equals(uri)) return;
+            reevaluate();
+        }
+    }
+}