[SPARK-22067][SQL] ArrowWriter should use position when setting UTF8String ByteBuffer

## What changes were proposed in this pull request?

The ArrowWriter StringWriter was setting Arrow data using a position of 0 instead of the actual position in the ByteBuffer.  This was currently working because of a bug ARROW-1443, and has been fixed as of
Arrow 0.7.0.  Testing with this version revealed the error in ArrowConvertersSuite test string conversion.

## How was this patch tested?

Existing tests, manually verified working with Arrow 0.7.0

Author: Bryan Cutler <[email protected]>

Closes apache#19284 from BryanCutler/arrow-ArrowWriter-StringWriter-position-SPARK-22067.

Author: BryanCutler

sql/core/src/main/scala/org/apache/spark/sql/execution/arrow/ArrowWriter.scala

@@ -234,8 +234,9 @@ private[arrow] class StringWriter(val valueVector: NullableVarCharVector) extend
 
   override def setValue(input: SpecializedGetters, ordinal: Int): Unit = {
     val utf8 = input.getUTF8String(ordinal)
+    val utf8ByteBuffer = utf8.getByteBuffer
     // todo: for off-heap UTF8String, how to pass in to arrow without copy?
-    valueMutator.setSafe(count, utf8.getByteBuffer, 0, utf8.numBytes())
+    valueMutator.setSafe(count, utf8ByteBuffer, utf8ByteBuffer.position(), utf8.numBytes())
   }
 }