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A new LocalArray C++ class lets us specify a "reasonable" amount of stack to
use, but transparently fall back to using the heap if we need more space.

The three places I've chosen to use LocalArray in this patch are fairly
random; all they have in common is that they're the places where we call
GetStringUTFRegion. There are more places LocalArray will be useful: the
java.io.File JNI in particular.

Bug: 2257819