native-task: Simplify ByteBufferDataReader/Writer

The ByteBufferDataReader and ByteBufferDataWriter class are more complex than necessary:

• several methods related to reading/writing strings and char arrays are implemented but never used by the native task code. Given that the use case for these classes is limited to serializing binary data to/from the native code, it seems unlikely people will want to use these methods in any performance-critical space. So, let's do simpler implementations that are less likely to be buggy, even if they're slightly less performant.
• methods like readLine() are even less likely to be used. Since it's a complex implementation, let's just throw UnsupportedOperationException
• in the test case, we can use Mockito to shorten the amount of new code