Update SubtitleWebView to use center alignment by default

This mimics the behaviour of SubtitleTextView for cues that don't have
alignment set.

PiperOrigin-RevId: 309389392

library/ui/src/main/java/com/google/android/exoplayer2/ui/SubtitleWebView.java

@@ -318,17 +318,16 @@ import java.util.List;
 
 
   private static String convertAlignmentToCss(@Nullable Layout.Alignment alignment) {
   private static String convertAlignmentToCss(@Nullable Layout.Alignment alignment) {
     if (alignment == null) {
     if (alignment == null) {
-      return "unset";
+      return "center";
     }
     }
     switch (alignment) {
     switch (alignment) {
       case ALIGN_NORMAL:
       case ALIGN_NORMAL:
         return "start";
         return "start";
-      case ALIGN_CENTER:
-        return "center";
       case ALIGN_OPPOSITE:
       case ALIGN_OPPOSITE:
         return "end";
         return "end";
+      case ALIGN_CENTER:
       default:
       default:
-        return "unset";
+        return "center";
     }
     }
   }
   }