Fix fallback behavior of UnsafeReflectionAllocator when AccessibleObject isn't so accessible

[rhernandez35]

I'm the lucky owner of a post-jigsaw Java platform that has restrictive security manager installed. Failing to catch the SecurityException thrown by a cross-classloader getDeclaredField call in UnsafeReflectionAllocator::getOverrideField prevents any Gson instances from being created.

I'm hoping the fallback behavior in this class will be enough for us to use Gson without having to loosen our security policy.

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[rhernandez35]

@googlebot I signed it!

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[eamonnmcmanus]

This looks reasonable. Thanks for the carefully-constructed test!