Usage of `Label` does not work when rule is defined as an external repo

[get_feature_module_paths()](https://github.com/bazelbuild/rules_android/blob/pre-alpha/rules/android_application/android_feature_module_rule.bzl#L85) is broken due to the usage of `Label()`. Creating a Label in an external repo will append `@external_repo_name//` to the label which is not correct.