Properly handle default_package_metadata attribute. (#1217)

* Update buildozer to account for the renaming the global attribute `applicable_licenses` to `package_metadata`.
The previous code assumed that `applicable_licenses` existed, and used
it to set the type for `default_applicable_licenses`.  Now we simply
assume that `default_applicable_licenses` and `default_package_metadata`
are label lists.

* Might as well do the attribute too