Suppress an unchecked cast warning in ShimPluginRegistry (#9145)

shell/platform/android/io/flutter/embedding/engine/plugins/shim/ShimPluginRegistry.java

@@ -68,6 +68,7 @@ public class ShimPluginRegistry implements PluginRegistry {
   }
 
   @Override
+  @SuppressWarnings("unchecked")
   public <T> T valuePublishedByPlugin(String pluginKey) {
     return (T) pluginMap.get(pluginKey);
   }