include previous key file contents when adding key. do not delete temp files, since they will be renamed to the auth file. if they're left around, the temp cleaner will get them

bubble-server/src/main/java/bubble/service/device/StandardFlexRouterService.java

@@ -243,8 +243,9 @@ public class StandardFlexRouterService extends SimpleDaemon implements FlexRoute
         if (Arrays.stream(lines).anyMatch(line -> line.trim().equals(trimmedKey))) {
             if (log.isDebugEnabled()) log.debug("allowKey: already present: "+trimmedKey);
         } else {
-            @Cleanup("delete") final File temp = temp("flex_keys_", ".tmp");
+            final File temp = temp("flex_keys_", ".tmp");
             final String dataToWrite = authFileContents != null && authFileContents.endsWith("\n") ? trimmedKey + "\n" : "\n" + trimmedKey + "\n";
+            toFileOrDie(temp, authFileContents, true);
             toFileOrDie(temp, dataToWrite, true);
             renameOrDie(temp, authFile);
             if (log.isInfoEnabled()) log.info("allowKey: added key: "+trimmedKey);
@@ -267,7 +268,7 @@ public class StandardFlexRouterService extends SimpleDaemon implements FlexRoute
             }
         }
         b.append("\n");
-        @Cleanup("delete") final File temp = temp("flex_keys_", ".tmp");
+        final File temp = temp("flex_keys_", ".tmp");
         toFileOrDie(temp, b.toString());
         renameOrDie(temp, authFile);
         if (found) {