Fix TODO in Boxes.java

The fix is as per MP4 spec ISO/IEC 14496-12

PiperOrigin-RevId: 518240043

testdata/src/test/assets/muxerdumps/mp4_with_float_metadata.mp4.dump

@@ -14,7 +14,7 @@ moov (764 bytes):
       Data = length 28, hash 2BFD5C32
   trak (534 bytes):
     tkhd (92 bytes):
-      Data = length 84, hash C25E979A
+      Data = length 84, hash 8893F5BB
     mdia (434 bytes):
       mdhd (32 bytes):
         Data = length 24, hash 50217AD

testdata/src/test/assets/muxerdumps/mp4_with_string_metadata.mp4.dump

@@ -14,7 +14,7 @@ moov (778 bytes):
       Data = length 42, hash F4AA183B
   trak (534 bytes):
     tkhd (92 bytes):
-      Data = length 84, hash C25E979A
+      Data = length 84, hash 8893F5BB
     mdia (434 bytes):
       mdhd (32 bytes):
         Data = length 24, hash 50217AD

testdata/src/test/assets/muxerdumps/mp4_with_xmp.mp4.dump

@@ -7,7 +7,7 @@ moov (650 bytes):
     Data = length 100, hash A5ADE288
   trak (534 bytes):
     tkhd (92 bytes):
-      Data = length 84, hash C25E979A
+      Data = length 84, hash 8893F5BB
     mdia (434 bytes):
       mdhd (32 bytes):
         Data = length 24, hash 50217AD

testdata/src/test/assets/muxerdumps/video_track_tkhd_box.dump

 tkhd (92 bytes):
-  Data = length 84, hash A5B8C30C
+  Data = length 84, hash 6BEE212D