extract mp3 bitrate from synchronized header

[chientrm]

This helps determining bitrate of the first frame synchronized.

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[icbaker]

As explained in google/ExoPlayer#11124 (comment) we won't accept this into the library, because it assumes all MP3 files are CBR, which is not a valid assumption. Propagating the bitrate in a way that either works for both VBR and CBR, or only affects truly CBR files is much harder as described in that comment.

My suggestion in google/ExoPlayer#11124 (comment) is that if you want to make this assumption you can make (and keep) this change locally.