Disabled state UI of Flash Notifications preview button

When both the camera flash and screen flash toggles are turned off, tapping on the preview button will have no effect and may confuse users. To avoid this, the appearance of the preview button should be updated to clearly indicated that its current state is disabled. This wil help users better understand the situation and prevent confusion.

Bug: 276494146
Test: checks the UI manually
Test: make RunSettingsRoboTests ROBOTEST_FILTER=FlashNotificationsPreviewPreferenceTest
Change-Id: I55b11188fde6e551921a9b0f7c89daa20a8b766b