Downloading and caching the Google Maps assets is against the Google Maps API TOS.
Check the TOS section 8:
License Restrictions. Except as expressly permitted under the Terms, or unless you have received prior written authorization from Google (or, as applicable, from the provider of particular Content), the license granted to you in Section 7 is conditioned on your adherence to all of the restrictions in this Section 8. Under this Section 8, you must not (nor may you permit anyone else to):
8.2. copy, translate, modify, create a derivative work of, pre-fetch, cache, or publicly display any Content or any part thereof.