soong_config: bool_variables shortcut

Using a lot of boolean variables can become very verbose without adding
really any new information:

      variables: ["a", "b", "c"],
  }

  soong_config_bool_variable {
      name: "a",
  }

  soong_config_bool_variable {
      name: "b",
  }

  soong_config_bool_variable {
      name: "c",
  }

Now turns into:

      bool_variables: ["a", "b", "c"],
  }

Bug: 153161144
Test: built-in tests
Change-Id: If5455a38433431c7ecbce1e5b32cfbb47f42602a
Merged-In: If5455a38433431c7ecbce1e5b32cfbb47f42602a
(cherry picked from commit 2b8b89cf)