Update importMaven to start fetching signature files
Update importMaven to start fetching signature files

This is in preparation of enabling Gradle signature verification.
These .asc files will be used to validate that dependencies were
built the by trusted authors.

Test: ./development/importMaven/import_maven_artifacts.py -n junit:junit4:4.13
Change-Id: I60bf57f8c59076aec55d2758ff130eba510245ea

AndroidX/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx