Fix FirstUniqueStrings after conversion to generics

The conversion of FirstUniqueStrings to be implemented on top of the
generic firstUnique accidentally used a different threshold to switch
from the list implementation to the map implementation.  Modify the
threshold of firstUnique to match the old value from FirstUniqueStrings
now that it doesn't have the reflection overhead.  While we're at it,
also make firstUnique make a copy of the list, and make FirstUniqueStrings
a pure wrapper around firstUnique.

Test: BenchmarkFirstUniqueStrings
Change-Id: Icc2febea663142c508ff2e4be65a8a68121631d5