While we're in there, rename the underlying field from categoryId to
labelId, including renaming the proto field. This is cleanup following
from I5df6f0c5. At that time, we opted to leave the name the same
because Megastore had problems with renaming proto fields and hence we
couldn't easily rename @Column fields. Now that ReviewDb (including the
Megastore implementation) is gone, this argument no longer applies.
See I6982fb24 for context.
Change-Id: I011e0bfeb74345a20b5eb0bb97ce61de7cb12bd0
c144e78d78