RFR 8247274: (test) HexPrinter cleanup

[Roger Riggs]

Please review cleanup to the test support for 
jdk.test.lib.hexprinter.HexPrinter.

Correct issues with the test support for jdk.test.lib.hexdump.HexPrinter.
  - Correct the swapped definitions of Formatters.ASCII and PRINTABLE
    Printable should always return a single character,
    ASCII provides mnemonics to be consistent with the command line 
hexdump.
- For the methods to format ByteBuffers align the descriptions of the 
index and length to match ByteBuffer.
  - Correctly implement the extraction of bytes from the ByteBuffer
  - a newline in the annotation should print as '\n' instead of causing 
a line break

Webrev:
     http://cr.openjdk.java.net/~rriggs/webrev-hexprinter-8247274-1/
Issue:
    https://bugs.openjdk.java.net/browse/JDK-8247274

Thanks, Roger