[SPARK-9725] [SQL] fix serialization of UTF8String across different JVM

The BYTE_ARRAY_OFFSET could be different in JVM with different configurations (for example, different heap size, 24 if heap > 32G, otherwise 16), so offset of UTF8String is not portable, we should handler that during serialization.

Author: Davies Liu <davies@databricks.com>

Closes #8210 from davies/serialize_utf8string.