Insert dynamic dashboard tiles into optional placeholders

Insteads of inserting tile onto screen using absolute priority values,
now each page can have a placeholder preference, and at run time we will
add dynamic dashboard tiles to placeholder's place.

Bug: 32827787
Test: RunSettingsRoboTests
Change-Id: I1fe9e11dce4eb6fb4a9b56af05a2b8e5cdae00d2

src/com/android/settings/dashboard/DashboardTilePlaceholderPreferenceController.java

@@ -0,0 +1,68 @@
+/*
+ * 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 com.android.settings.dashboard;
+
+import android.content.Context;
+import android.support.v7.preference.Preference;
+import android.support.v7.preference.PreferenceScreen;
+
+import com.android.settings.core.PreferenceController;
+
+/**
+ * PreferenceController for a dashboard_tile_placeholder, a special preference marking where
+ * dynamic dashboard tiles should be injected in a screen. It is optional when building
+ * preference screen in xml. If not present, all dynamic dashboard tiles will be added to the
+ * bottom of page.
+ */
+class DashboardTilePlaceholderPreferenceController extends PreferenceController {
+
+    private static final String KEY_PLACEHOLDER = "dashboard_tile_placeholder";
+
+    private int mOrder = Preference.DEFAULT_ORDER;
+
+    public DashboardTilePlaceholderPreferenceController(Context context) {
+        super(context);
+    }
+
+    @Override
+    public void displayPreference(PreferenceScreen screen) {
+        final Preference pref = screen.findPreference(getPreferenceKey());
+        if (pref != null) {
+            mOrder = pref.getOrder();
+            screen.removePreference(pref);
+        }
+    }
+
+    @Override
+    public boolean isAvailable() {
+        return false;
+    }
+
+    @Override
+    public boolean handlePreferenceTreeClick(Preference preference) {
+        return false;
+    }
+
+    @Override
+    public String getPreferenceKey() {
+        return KEY_PLACEHOLDER;
+    }
+
+    public int getOrder() {
+        return mOrder;
+    }
+}