I have a ContentProvider class and is declared in AndroidMenifest.xml like this:
Proguard excludes all inherited content providers by default with this line (make sure it's in your cfg):
-keep public class * extends android.content.ContentProvider
If you have any additional inheritance you should exclude it as well or exclude your specific Content Provider class, for example:
-keep public class org.iii.romulus.meridian.MediaSearchProvider