ConcurrentHashMap: update API for keySet.

On Android M and earlier, keySet() returned a Set<K>. Changing the
return type to a type that doesn't exist on previous releases is an
incompatible change. We therefore revert it so that applications that
compile against the Android N SDK will continue to work on older
releases.

Note that there is no API update that accompanies this change. Due to
an asymmetry in doclava, this method is absent from the *api.txt files
but *does* exist in the stubs.

bug: 28099367
Change-Id: Ife90592847de1752cd33efdb2e7e0ec082df42fc