Migrate Anvil away from AGP's legacy variant API

[hungvietnguyen]

In AGP 9.0 (canaries coming soon), access to the old variant API will be disabled by default (AGP roadmap), so it would be great to migrate Anvil away from the legacy variant API.

Related bug in KGP: https://youtrack.jetbrains.com/issue/KT-77300/

Using the legacy variant API not only will break users in AGP 9.0, but currently it already breaks users who are using test fixtures (ZacSweers#89).

For an inspiration to get started, please see google/ksp#2250 where some progress has been made to migrate KSP.