Add fLaC prefix to FLAC initialization data

The fLaC prefix is included in the initialization data output
from the MKV extractor, so this is highly likely ot be the
right thing to do.

Issue: #6397
PiperOrigin-RevId: 268244365

library/core/src/main/java/com/google/android/exoplayer2/extractor/mp4/AtomParsers.java

@@ -1162,7 +1162,16 @@ import java.util.List;
         System.arraycopy(opusMagic, 0, initializationData, 0, opusMagic.length);
         System.arraycopy(opusMagic, 0, initializationData, 0, opusMagic.length);
         parent.setPosition(childPosition + Atom.HEADER_SIZE);
         parent.setPosition(childPosition + Atom.HEADER_SIZE);
         parent.readBytes(initializationData, opusMagic.length, childAtomBodySize);
         parent.readBytes(initializationData, opusMagic.length, childAtomBodySize);
-      } else if (childAtomType == Atom.TYPE_dfLa || childAtomType == Atom.TYPE_alac) {
+      } else if (childAtomType == Atom.TYPE_dfLa) {
+        int childAtomBodySize = childAtomSize - Atom.FULL_HEADER_SIZE;
+        initializationData = new byte[4 + childAtomBodySize];
+        initializationData[0] = 0x66; // f
+        initializationData[1] = 0x4C; // L
+        initializationData[2] = 0x61; // a
+        initializationData[3] = 0x43; // C
+        parent.setPosition(childPosition + Atom.FULL_HEADER_SIZE);
+        parent.readBytes(initializationData, /* offset= */ 4, childAtomBodySize);
+      } else if (childAtomType == Atom.TYPE_alac) {
         int childAtomBodySize = childAtomSize - Atom.FULL_HEADER_SIZE;
         int childAtomBodySize = childAtomSize - Atom.FULL_HEADER_SIZE;
         initializationData = new byte[childAtomBodySize];
         initializationData = new byte[childAtomBodySize];
         parent.setPosition(childPosition + Atom.FULL_HEADER_SIZE);
         parent.setPosition(childPosition + Atom.FULL_HEADER_SIZE);