Use constants from StandardCharsets instead of hard-coded strings

Use constants from StandardCharsets instead of hard-coded strings

Instead of hard-coding the charset strings "US-ASCII", "UTF-8", and
"ISO-8859-1", use the corresponding constants from StandardCharsets.

UnsupportedEncodingException is not thrown when the StandardCharset
constants are used, so remove the now redundant handling.

Because the encoding names are no longer hard-coded strings, also
remove redundant $NON-NLS warning suppressions.

Also replace existing usages of the constants with static imports.

Change-Id: I0a4510d3d992db5e277f009a41434276f95bda4e
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>