R8 Proguard mappings are not propagated to the resource shrinker

Currently there is no way to pass proguard mappings to the resource shrinker. Without it a bunch of resource that should that should be kept are removed instead. This is currently not supported directly by the shrinker tool. I've filed a[ feature request on android](https://partnerissuetracker.corp.google.com/issues/299961324) requesting support, but if the feature is not accepted we will need to implement our own wrapper.