Replace Proguard rules with @Keep annotations

[zugaldia]

Currently, to define our minification rules we use a proguard-rules.pro file, which is error-prone because it needs to be manually updated with every release (e.g. #8944) and it's not directly connected with the code, making it easier to forget to include classes that need to be tracked.

Instead, we should explore replacing all or some of those rules with the @Keep annotation, which gets attached to its target and it's easier to update incrementally.

cc: @tobrun @Guardiola31337