Add non-ascii characters to SpannedToHtmlConverterTest

These are handled by converting to ampersand codes - add a test to show
that's explicit & deliberate.

PiperOrigin-RevId: 304163712

library/ui/src/test/java/com/google/android/exoplayer2/ui/SpannedToHtmlConverterTest.java

@@ -147,6 +147,16 @@ public class SpannedToHtmlConverterTest {
   }
 
   @Test
+  public void convert_convertsNonAsciiCharactersToAmpersandCodes() {
+    String html =
+        SpannedToHtmlConverter.convert(
+            new SpannableString("Strìng with 優しいの non-ASCII characters"));
+
+    assertThat(html)
+        .isEqualTo("Strìng with 優しいの non-ASCII characters");
+  }
+
+  @Test
   public void convert_ignoresUnrecognisedSpan() {
     SpannableString spanned = new SpannableString("String with unrecognised span");
     spanned.setSpan(