Added constant for "empty speak tag" (#835)

* Added constant for Empty Speak Tag

* Fix Checkstyle issues

Author: LeeParrishMSFT

libraries/bot-schema/src/main/java/com/microsoft/bot/schema/SpeechConstants.java

@@ -0,0 +1,21 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+package com.microsoft.bot.schema;
+
+/**
+ * Defines constants that can be used in the processing of speech interactions.
+ */
+public final class SpeechConstants {
+
+    private SpeechConstants() {
+
+    }
+
+     /**
+     * The xml tag structure to indicate an empty speak tag, to be used in the 'speak' property of an Activity.
+     * When this is set it indicates to the channel that speech should not be generated.
+     */
+    public static final String EMPTY_SPEAK_TAG =
+        "<speak version=\"1.0\" xmlns=\"https://www.w3.org/2001/10/synthesis\" xml:lang=\"en-US\" />";
+}