Add a plus button to the mobile pref on Network & internet page

On the Network & internet page, we have a "Mobile network" pref that in
single-SIM mode leads to a detail page for the current SIM. In multi-SIM
mode it has more complicated behavior (leading either to details about
the current subscription if there is only one, or a list of
subscriptions if more than one).

One of the things we wanted to add was a shortcut to add another eSIM
subscription. So this CL adds a plus button control on the right of the
preference which leads to a flow to add another mobile subscription via
the eSIM manager.

Bug: 116349402
Test: make RunSettingsRoboTests
Change-Id: I38e0031e3bd603e93c45dcb4557750e7bc1b8b5a

src/com/android/settings/widget/AddPreference.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright (C) 2019 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 com.android.settings.widget;
+
+import android.content.Context;
+import android.util.AttributeSet;
+import android.view.View;
+
+import com.android.settings.R;
+import com.android.settingslib.RestrictedPreference;
+
+import androidx.annotation.VisibleForTesting;
+import androidx.preference.PreferenceViewHolder;
+
+/**
+ * A preference with a plus button on the side representing an "add" action. The plus button will
+ * only be visible when a non-null click listener is registered.
+ */
+public class AddPreference extends RestrictedPreference implements View.OnClickListener {
+
+    private OnAddClickListener mListener;
+    private View mWidgetFrame;
+
+    public AddPreference(Context context, AttributeSet attrs) {
+        super(context, attrs);
+    }
+
+    @VisibleForTesting
+    int getAddWidgetResId() {
+        return R.id.add_preference_widget;
+    }
+
+    /** Sets a listener for clicks on the plus button. Passing null will cause the button to be
+     * hidden. */
+    public void setOnAddClickListener(OnAddClickListener listener) {
+        mListener = listener;
+       if (mWidgetFrame != null) {
+           mWidgetFrame.setVisibility(shouldHideSecondTarget() ? View.GONE : View.VISIBLE);
+       }
+    }
+
+    @Override
+    protected int getSecondTargetResId() {
+        return R.layout.preference_widget_add;
+    }
+
+    @Override
+    protected boolean shouldHideSecondTarget() {
+        return mListener == null;
+    }
+
+    @Override
+    public void onBindViewHolder(PreferenceViewHolder holder) {
+        super.onBindViewHolder(holder);
+        mWidgetFrame = holder.findViewById(android.R.id.widget_frame);
+        final View addWidget = holder.findViewById(getAddWidgetResId());
+        addWidget.setEnabled(true);
+        addWidget.setOnClickListener(this);
+    }
+
+    @Override
+    public void onClick(View view) {
+        if (view.getId() == getAddWidgetResId() && mListener != null) {
+            mListener.onAddClick(this);
+        }
+    }
+
+    public interface OnAddClickListener {
+        void onAddClick(AddPreference p);
+    }
+}