Speed up DecimalFormatSymbols.

Speed up DecimalFormatSymbols.

We don't need to create temporary String objects; we can just pass a char
directly. We also don't need to initialize aspects of our native peer if
we know we're going to overwrite them straight away, and making copying
into ICU the responsibility of the icu4jni class rather than the java.text
is slightly cleaner.

Together, these changes make creating a new NumberFormat about 20% faster.