auditing, caching proxy on the host for `fdroid build`

@uniqx and I were discussing yesterday a way to integrate our verification processes into the Android SDK tools. We came up with an idea to make fdroid build launch its own caching proxy, which would then verify all downloads before caching them and making them available.

• gradle has proxy options.

• sdkmanager has options to use HTTP and a proxy

Turns out there is another build cache from the android tools, but it seems configurable enough for us to handle it. But that mostly seems to cache the pre-dexing, so perhaps we don't even want that. https://developer.android.com/studio/build/build-cache.html