Final UX for Wi-Fi assistant platform settings.

When one app is available, the toggle works as before. When multiple
apps are available, we allow the user to choose the app in a dialog
before turning on the setting.

Bug: 13780935
Change-Id: I1c4391bf97a53febe580fb2b896b4850372062e7

src/com/android/settings/AppListSwitchPreference.java

@@ -0,0 +1,59 @@
+package com.android.settings;
+
+import android.content.Context;
+import android.os.Bundle;
+import android.util.AttributeSet;
+import android.util.Log;
+import android.view.View;
+import android.widget.Checkable;
+
+/**
+ * A hybrid of AppListPreference and SwitchPreference, representing a preference which can be on or
+ * off but must have a selected value when turned on.
+ *
+ * It is invalid to show this preference when zero valid apps are present.
+ */
+public class AppListSwitchPreference extends AppListPreference {
+    private static final String TAG = "AppListSwitchPref";
+
+    private Checkable mSwitch;
+
+    public AppListSwitchPreference(Context context, AttributeSet attrs) {
+        super(context, attrs, 0, R.style.AppListSwitchPreference);
+    }
+
+    @Override
+    protected void onBindView(View view) {
+        super.onBindView(view);
+        mSwitch = (Checkable) view.findViewById(com.android.internal.R.id.switchWidget);
+        mSwitch.setChecked(getValue() != null);
+    }
+
+    @Override
+    protected void showDialog(Bundle state) {
+        if (getValue() != null) {
+            // Turning off the current value.
+            if (callChangeListener(null)) {
+                setValue(null);
+            }
+        } else if (getEntryValues() == null || getEntryValues().length == 0) {
+            Log.e(TAG, "Attempting to show dialog with zero entries: " + getKey());
+        } else if (getEntryValues().length == 1) {
+            // Suppress the dialog and just toggle the preference with the only choice.
+            String value = getEntryValues()[0].toString();
+            if (callChangeListener(value)) {
+                setValue(value);
+            }
+        } else {
+            super.showDialog(state);
+        }
+    }
+
+    @Override
+    public void setValue(String value) {
+        super.setValue(value);
+        if (mSwitch != null) {
+            mSwitch.setChecked(value != null);
+        }
+    }
+}