Handle enabled: false via conditions_default

In this Android.bp file
```
my_cc_defaults {
  enabled: false,
  soong_config_variables: {
    my_bool_variable: {
       conditions_default: {enabled: false},
    }
  }
}
```
The inner enabled: false is a no-op because the top-level enabled is
false. Currently, bp2build will raise an exception for this Android.bp
file.

However, it does not need to. `productVariableConfigEnableLabels` runs
only if the top-level enabled is false. If it sees enabled: false via
conditions_default, it should just ignore it since it is a no-op.

Test: go test ./bp2build
Bug: 210546943
Change-Id: I816f209eaf21de65ddfbc2893e5255be94bcaa11