Fix invalid cast in LowApiPermissionsHelper (#201)

Author: eavsievich

easypermissions/src/main/java/pub/devrel/easypermissions/helper/LowApiPermissionsHelper.java

@@ -41,7 +41,7 @@ public Context getContext() {
         } else if (getHost() instanceof Fragment) {
             return ((Fragment) getHost()).getContext();
         } else if (getHost() instanceof android.app.Fragment) {
-            return ((Fragment) getHost()).getContext();
+            return ((android.app.Fragment) getHost()).getActivity();
         } else {
             throw new IllegalStateException("Unknown host: " + getHost());
         }