[SPARK-50155][3.5] Move scala and java files to their default folders

### What changes were proposed in this pull request?
The pr aims to move `scala` and `java` files to their default folders (`src/main/scala` and `src/main/java`), includes:
- `ByteArrayUtils.java`
  from: `common/utils/src/main/scala/org/apache/spark/unsafe/array/ByteArrayUtils.java`
  to: `common/utils/src/main/java/org/apache/spark/unsafe/array/ByteArrayUtils.java`
- `CustomDecimal.scala`
  from: `connector/avro/src/main/java/org/apache/spark/sql/avro/CustomDecimal.scala`
  to: `connector/avro/src/main/scala/org/apache/spark/sql/avro/CustomDecimal.scala`

PS: The pr is backport branch-3.5, master pr is: apache#48691
Note: In branch-3.5, the `OrcCompressionCodec.java` file does not exist, so there is no need to move it.

### Why are the changes needed?
Move `scala` and `java` files to the default folder of the project to avoid `misunderstandings` for spark developers.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Pass GA.

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes apache#48700 from panbingkun/SPARK-50155_branch3.5.

Authored-by: panbingkun <[email protected]>
Signed-off-by: Max Gekk <[email protected]>