This is a follow-up to Lai's refactoring of the get reservation types
code to return a set rather than a single type. Since we're always
returning a set now, the more natural way to represent a label that is
not reserved is to return an empty set rather than a set containing
UNRESERVED.
Also fixes some minor style issues I ran across regarding static
importing and test method naming that I ran across (no logic
implications).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=151132116