fix: FRMA generation (#8367)

Author: tykus160

lib/util/mp4_generator.js

@@ -833,7 +833,7 @@ shaka.util.Mp4Generator = class {
    * @private
    */
   frma_(codecs) {
-    const codec = codecs.substring(0, codecs.indexOf('.'));
+    const codec = codecs.split('.')[0];
     const Mp4Generator = shaka.util.Mp4Generator;
     const codecNumber = this.stringToCharCode_(codec);
     const bytes = new Uint8Array([