Proguard configs should be included from .aar's

[Redth]

.AAR files can include proguard.txt configuration files.

These are not currently being seen and included in the proguard/r8 usage during the build.

Probably ResolveLibraryProjectImports needs to be responsible for looking for these after extracting the .aar's and adding them to the @(ProguardConfiguration) group.

We currently ship proguard.txt files inside some nuget packages with a .targets file which includes them in the ProguardConfiguration item group, so it would be important to test what happens if duplicate config files are passed to proguard/r8 if this work is done (since there will be nuget packages with them we cannot change).