RFR JDK-8073924: Update test/java/nio/charset/Charset/NIOCharsetAvailability.java to work with module system

[Xueming Shen]

Please help review the change for JDK-8073924.

issue: https://bugs.openjdk.java.net/browse/JDK-8073924
webrev: http://cr.openjdk.java.net/~sherman/8073924/webrev

Thanks,
-Sherman