Skip creating variants for disabled OSes

The documentation java_genrule_host states that it creates a single
variant, which would make it work with the single variant fallback
in AddDependency used by the data property, but it actually has a host
and a host-cross windows variant.  Modify osMutator to take the
OS-specific enabled properties into account to skip creating variants
that will immediately be disabled so there is a single variant.

Test: m checkbuild
Change-Id: Ic2daab29f4fa3a3797d7a08348fbfcf1036ec5dc