Update the FlashNotificationsPreferenceFragmentTest to return correct

metric category

**Root Cause**
The test was written when the metric category is set to 0. After we
update the metric category to FLASH_NOTIFICATION_SETTINGS in the
FlashNotificationsPrefereenceFragment, we forgot to update the test.

Bug: 279082331
Test: atest FlashNotificationsPreferenceFragmentTest
Change-Id: Icd709bd9e571ca264226d0ca860e5c482eae3927