For each sdk built generate a JSON file describing its contents

Some build scripts need to know information about the contents of an
sdk, such as what APIs it provides (via java_sdk_library). Rather than
duplicate that information in the scripts or attempt to access that
information (where available) by looking at the contents of the
snapshot this change generates a JSON file that sits alongside the
snapshot itself.

The info file can be generated without generating the snapshot zip file
but whenever a snapshot zip file is generated the info is generated
too. The info file sits alongside the zip file in out/mainline-sdks.

Bug: 204763318
Test: m art-module-sdk
      m dist
Change-Id: I289530bb21693dc6443826c24c17c9b5d85d2d8b