JNI direct buffer function speedup, part 1.

JNI direct buffer function speedup, part 1.

This is a fairly straightforward improvement to GetDirectBufferAddress,
caching classes and methodIDs instead of looking them up on every call.
This is the best we can do without making the function more vulnerable
to internal changes to Harmony's NIO implementation.

It looks like we need to take this farther, but this way we have a
relatively fast and relatively safe version to fall back on.

For internal bug 1943379.