Add Feature Flag for Hearing Aid Profile

Using the Settings App-Developer Options-Feature Flag, allow the user to
enable or disable the Hearing Aid Profile.

Test: Manual testing using Settings App
Change-Id: I16b51d7feabc914219c24731eb39a23bd1782571

src/com/android/settings/development/featureflags/FeatureFlagPersistent.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2018 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.development.featureflags;
+
+import android.content.Context;
+import android.os.SystemProperties;
+import android.text.TextUtils;
+import android.util.FeatureFlagUtils;
+import android.util.Log;
+
+import com.android.settings.core.FeatureFlags;
+
+import java.util.HashSet;
+
+import androidx.annotation.VisibleForTesting;
+
+/**
+ * Helper class to get feature persistent flag information.
+ */
+public class FeatureFlagPersistent {
+    private static final HashSet<String> PERSISTENT_FLAGS;
+    static {
+        PERSISTENT_FLAGS = new HashSet<>();
+        PERSISTENT_FLAGS.add(FeatureFlags.HEARING_AID_SETTINGS);
+    }
+
+    public static boolean isEnabled(Context context, String feature) {
+        String value = SystemProperties.get(FeatureFlagUtils.PERSIST_PREFIX + feature);
+        if (!TextUtils.isEmpty(value)) {
+            return Boolean.parseBoolean(value);
+        } else {
+            return FeatureFlagUtils.isEnabled(context, feature);
+        }
+    }
+
+    public static void setEnabled(Context context, String feature, boolean enabled) {
+        SystemProperties.set(FeatureFlagUtils.PERSIST_PREFIX + feature, enabled ? "true" : "false");
+        FeatureFlagUtils.setEnabled(context, feature, enabled);
+    }
+
+    public static boolean isPersistent(String feature) {
+        return PERSISTENT_FLAGS.contains(feature);
+    }
+
+    /**
+     * Returns all persistent flags in their raw form.
+     */
+    @VisibleForTesting(otherwise = VisibleForTesting.NONE)
+    static HashSet<String> getAllPersistentFlags() {
+        return PERSISTENT_FLAGS;
+    }
+}
+