RFR: Update java/nio/charset/Charset/NIOCharsetAvailabilityTest.java to eliminate dependency on sun.misc.Launcher

[Amy Lu]

jdk/test/java/nio/charset/Charset/NIOCharsetAvailabilityTest.java has 
dependency on sun.misc.Launcher
This fix is to remove above internal API dependency from the test.

bug: https://bugs.openjdk.java.net/browse/JDK-8066131
webrev: http://cr.openjdk.java.net/~weijun/8066131/webrev.00/

Thanks,
Amy