[I] `UnionListWriter#startList` points the inner writer at the wrong position [arrow-java]

[via GitHub]

nbauernfeind opened a new issue, #585:
URL: https://github.com/apache/arrow-java/issues/585

   ### Describe the bug, including details regarding any error messages, 
version, and platform.
   
   UnionListWriter's impl:
   ```java
     @Override
     public void startList() {
       vector.startNewValue(idx());
       writer.setPosition(vector.getOffsetBuffer().getInt((idx() + 1L) * 
OFFSET_WIDTH));
       listStarted = true;
     }
   ```
   
   
   Whereas UnionLargeListViewWriter:
   ```java
     @Override
     public void startList() {
       vector.startNewValue(idx());
       writer.setPosition(vector.getOffsetBuffer().getInt((idx()) * 
OFFSET_WIDTH));
       listStarted = true;
     }
   ```
   
   Note that the issue is that `UnionListWriter` fetches the offset of `idx() + 
1` -- not the offset of `idx()` like the Large impl.
   
   Also note that `startNewValue` returns the position that the writer can be 
set to; we don't need to poke into the offset buffer. 
   


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