fix(HLS): IMSC1 subtitles not working in a HLS stream (#4942)

Fixes #4350

The current code initially parses the subtitle track as text/vtt
(mimetype) since it has no codec. Subsequently, the stpp.ttml.im1t codec
is assigned but the mimetype of text/vtt is not changed to
application/mp4. This PR changes this so that once the codec is
assigned, the mimetype is recalculated.

lib/hls/hls_parser.js

@@ -945,6 +945,8 @@ shaka.hls.HlsParser = class {
       }
     });
 
+    const type = shaka.util.ManifestParserUtils.ContentType.TEXT;
+
     // Set the codecs for text streams.
     for (const tag of subtitleTags) {
       const groupId = tag.getRequiredAttrValue('GROUP-ID');
@@ -954,6 +956,9 @@ shaka.hls.HlsParser = class {
         if (textStreamInfos) {
           for (const textStreamInfo of textStreamInfos) {
             textStreamInfo.stream.codecs = codecs;
+            textStreamInfo.stream.mimeType =
+                this.guessMimeTypeBeforeLoading_(type, codecs) ||
+                this.guessMimeTypeFallback_(type);
           }
         }
       }