Explain BackgroundTaskSchedulerExternalUma otherwise visibility to lint

BackgroundTaskSchedulerExternalUma#toUmaEnumValueFromTaskId is used by
child classes but gives a lint warning. Fix the lint warning by
explaining to lint it was supposed to be protected not private.

Change-Id: I1f1cf080cf0750aaea1fdf94b1972b0f70e134b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2514963
Commit-Queue: Peter Wen <wnwen@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823600}

components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskSchedulerExternalUma.java

@@ -95,7 +95,7 @@ public abstract class BackgroundTaskSchedulerExternalUma {
         }
     }
 
-    @VisibleForTesting
+    @VisibleForTesting(otherwise = VisibleForTesting.PROTECTED)
     public static int toUmaEnumValueFromTaskId(int taskId) {
         switch (taskId) {
             case TaskIds.TEST: