[java] Fix avro bytes conversion for BQ storage

[RustedBones]

Avro bytes are represented with java ByteBuffer.

Current implementation expects a byte array. This fails in practice with a ClassCastException:
class java.nio.HeapByteBuffer cannot be cast to class [B

[github-actions]

Assigning reviewers. If you would like to opt out of this review, comment assign to next reviewer:

R: @Abacn for label java.
R: @damondouglas for label io.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).