don't cache decoded message over configurations

Since 773600c717, the message decoding
process might actually depend on theming parameters.  The MessageList
activity assumes that the theme will be reapplied during a call to
`recreate()`, so we better drop that cache on configuration changes.

k9mail/src/main/java/com/fsck/k9/activity/MessageLoaderHelper.java

@@ -146,6 +146,8 @@ public class MessageLoaderHelper {
      * asyncStartOrResumeLoadingMessage in a new instance of this class. */
     @UiThread
     public void onDestroyChangingConfigurations() {
+        cancelAndClearDecodeLoader();
+
         if (messageCryptoHelper != null) {
             messageCryptoHelper.detachCallback();
         }