This only adds support for a subset of Play Asset Delivery: this causes a single install-time asset pack to always be present, but doesn't add support for dynamically downloaded asset packs.
This is done by providing API access to app specific directories which don't have any limitations and allows us to bump the target sdk version to 30.
In addition, we're also bumping the min sdk version to 19 as version 18 is no longer supported by Google Play Services and only account of 0.3% of Android devices.
