Support ASR TTS fallback (#713)

* asr/tts fallback

* add notification

* add notification

* wip

* wip

* wip

* wip

* wip

* wip

* wip

* wip

lib/session/call-session.js

@@ -188,6 +188,24 @@ class CallSession extends Emitter {
     this._synthesizer = synth;
   }
 
+  /**
+   * ASR TTS fallback
+   */
+  get hasFallbackAsr() {
+    return this._hasFallbackAsr || false;
+  }
+
+  set hasFallbackAsr(i) {
+    this._hasFallbackAsr = i;
+  }
+
+  get hasFallbackTts() {
+    return this._hasFallbackTts || false;
+  }
+
+  set hasFallbackTts(i) {
+    this._hasFallbackTts = i;
+  }
   /**
    * default vendor to use for speech synthesis if not provided in the app
    */
@@ -1832,6 +1850,7 @@ Duration=${duration} `
   /**
    * called when the caller has hung up.  Provided for subclasses to override
    * in order to apply logic at this point if needed.
+   * return true if success fallback, return false if not
    */
   _callerHungup() {
     assert(false, 'subclass responsibility to override this method');