RFR: 8054214: JapaneseEra.getDisplayName doesn't return names if it's an additional era

[Masayoshi Okutsu]

Hi,

Please review the fix for JDK-8054214. It was necessary to override 
Era::getDisplayName to get era names from the specified property. This 
one fixes getAbbreviation() as well.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8054214

Webrev:
http://cr.openjdk.java.net/~okutsu/9/8054214/webrev.00/

Thanks,
Masayoshi