Extract counting of installed apps into helper class

This CL extracts the code that counts the number of installed apps
for a set of users from ManageApplications to a helper class,
InstalledAppCounter. The new helper is designed to be unit-testable
(PackageManager dependencies are injected via a new PackageManagerWrapper
abstraction) and customizable (the set of users for which to count is
specified by subclasses).

The CL adds a unit test for the InstalledAppCounter as well.

Test: make RunSettingsRoboTests
Bug: 32692748
Change-Id: Ie7fbd40d9dd998b27f1f9de227b6a187b5c9d315

src/com/android/settings/applications/PackageManagerWrapper.java

@@ -0,0 +1,45 @@
+/*
+ * 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.applications;
+
+import android.content.Intent;
+import android.content.pm.ApplicationInfo;
+import android.content.pm.ResolveInfo;
+
+import java.util.List;
+
+/**
+ * This interface replicates a subset of the android.content.pm.PackageManager (PM). The interface
+ * exists so that we can use a thin wrapper around the PM in production code and a mock in tests.
+ * We cannot directly mock or shadow the PM, because some of the methods we rely on are newer than
+ * the API version supported by Robolectric.
+ */
+public interface PackageManagerWrapper {
+    /**
+     * Calls {@code PackageManager.getInstalledApplicationsAsUser()}.
+     *
+     * @see android.content.pm.PackageManager.PackageManager#getInstalledApplicationsAsUser
+     */
+    List<ApplicationInfo> getInstalledApplicationsAsUser(int flags, int userId);
+
+    /**
+     * Calls {@code PackageManager.queryIntentActivitiesAsUser()}.
+     *
+     * @see android.content.pm.PackageManager.PackageManager#queryIntentActivitiesAsUser
+     */
+    List<ResolveInfo> queryIntentActivitiesAsUser(Intent intent, int flags, int userId);
+}