Support variable length sgpd boxes in fMPEG

For `sgpd` boxes with version >= 1, `the default_length` field is zero if the
length of the following sample group entries is variable.

Since the `FragmentedMp4Extractor` only supports one entry, skip the
`description_length` field if applicable (e.g. when version >= 1 and
`default_length` == 0), instead of throwing an error.

For parsing the following CencSampleEncryptionInformationGroupEntry, this should
make no difference.

Issue: #3177

Author: dimitry-unified-streaming

libraries/extractor/src/main/java/androidx/media3/extractor/mp4/FragmentedMp4Extractor.java

@@ -1477,18 +1477,17 @@ private static void parseSampleGroups(
     sgpd.setPosition(Mp4Box.HEADER_SIZE);
     int sgpdVersion = BoxParser.parseFullBoxVersion(sgpd.readInt());
     sgpd.skipBytes(4); // grouping_type == seig.
-    if (sgpdVersion == 1) {
-      if (sgpd.readUnsignedInt() == 0) {
-        throw ParserException.createForUnsupportedContainerFeature(
-            "Variable length description in sgpd found (unsupported)");
-      }
-    } else if (sgpdVersion >= 2) {
+    long default_length = sgpdVersion >= 1 ? sgpd.readUnsignedInt() : 0;
+    if (sgpdVersion >= 2) {
       sgpd.skipBytes(4); // default_sample_description_index.
     }
     if (sgpd.readUnsignedInt() != 1) { // entry_count.
       throw ParserException.createForUnsupportedContainerFeature(
           "Entry count in sgpd != 1 (unsupported).");
     }
+    if (sgpdVersion >= 1 && default_length == 0) {
+      sgpd.skipBytes(4); // description_length.
+    }
 
     // CencSampleEncryptionInformationGroupEntry
     sgpd.skipBytes(1); // reserved = 0.