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

RELEASENOTES.md

@@ -14,7 +14,8 @@
 * Add `HttpDataSource.getResponseCode` to provide the status code associated
   with the most recent HTTP response.
 * Fix initialization data handling for FLAC in MP4
-  ([#6396](https://github.com/google/ExoPlayer/issues/6396)).
+  ([#6396](https://github.com/google/ExoPlayer/issues/6396),
+  [#6397](https://github.com/google/ExoPlayer/issues/6397)).
 * Fix audio selection issue where languages are compared by bit rate
   ([#6335](https://github.com/google/ExoPlayer/issues/6335)).
 * Fix decoder selection for E-AC3 JOC streams

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

@@ -1148,7 +1148,16 @@ import java.util.List;
         System.arraycopy(opusMagic, 0, initializationData, 0, opusMagic.length);
         parent.setPosition(childPosition + Atom.HEADER_SIZE);
         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;
         initializationData = new byte[childAtomBodySize];
         parent.setPosition(childPosition + Atom.FULL_HEADER_SIZE);