Add accessibility usage preference to the Privacy screen.

Bug: 123595964
Test: Open Privacy page with one and two accessibility services
enabled and click the preference.
Test: Open Privacy page with zero accessibility services enabled and
do not see the preference.
Test: atest AccessibilityUsagePreferenceControllerTest

Change-Id: Ib1dab374cb53dfbc473b2d5ac49115e608a13ab8

src/com/android/settings/privacy/AccessibilityUsagePreferenceController.java

@@ -0,0 +1,55 @@
+/*
+ * 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.privacy;
+
+import android.accessibilityservice.AccessibilityServiceInfo;
+import android.content.Context;
+import android.view.accessibility.AccessibilityManager;
+
+import androidx.annotation.NonNull;
+import androidx.preference.Preference;
+
+import com.android.settings.R;
+import com.android.settings.core.BasePreferenceController;
+
+import java.util.List;
+
+
+public class AccessibilityUsagePreferenceController extends BasePreferenceController  {
+
+    private final @NonNull List<AccessibilityServiceInfo> mEnabledServiceInfos;
+
+    public AccessibilityUsagePreferenceController(Context context, String preferenceKey) {
+        super(context, preferenceKey);
+
+        final AccessibilityManager accessibilityManager = context.getSystemService(
+                AccessibilityManager.class);
+        mEnabledServiceInfos = accessibilityManager.getEnabledAccessibilityServiceList(
+                AccessibilityServiceInfo.FEEDBACK_ALL_MASK);
+    }
+
+    @Override
+    public int getAvailabilityStatus() {
+        return mEnabledServiceInfos.isEmpty() ? UNSUPPORTED_ON_DEVICE : AVAILABLE;
+    }
+
+    @Override
+    public CharSequence getSummary() {
+        return mContext.getResources().getQuantityString(R.plurals.accessibility_usage_summary,
+                mEnabledServiceInfos.size(), mEnabledServiceInfos.size());
+    }
+}