Inadequate keep rules with R8 full mode version 3.0

[christofferqa]

R8 version 3.0 started to remove generic signatures from items that are not matched by a -keep rule in full mode (see also https://r8.googlesource.com/r8/+/744d742137a82656b8ee27513f975e0528aecd78). The motivation for this change is to avoid that all generic signatures are retained in programs, when only a few are typically required for reflection.

As a result of this change, an extra -keep rule for retrofit2.Call is required to keep its generic signature.

I have opened PR #3579 to resolve this.