Add K-9 Mail warning toast

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/ui/DecryptActivity.java

@@ -162,6 +162,10 @@ public class DecryptActivity extends BaseActivity {
                     Uri uri = intent.getData();
                     if (uri != null) {
 
+                        if ("com.fsck.k9.attachmentprovider".equals(uri.getHost())) {
+                            Toast.makeText(this, R.string.error_reading_k9, Toast.LENGTH_LONG).show();
+                        }
+
                         if ("com.android.email.attachmentprovider".equals(uri.getHost())) {
                             Toast.makeText(this, R.string.error_reading_aosp, Toast.LENGTH_LONG).show();
                             finish();