Adds system settings related to flash notifications in settings provider

There are 2 types of flash notifications:
1. Camera flash notification
2. Screen flash notification

Adds 2 internal system settings keys for enabling these 2 kinds of flash notifications. An extra setting is introduced for configuring the color of screen flash notification.

Bug: 266775683
Test: make RunSettingsRoboTests ROBOTEST_FILTER=CameraFlashNotificationPreferenceControllerTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=ScreenFlashNotificationPreferenceControllerTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=FlashNotificationsPreviewPreferenceControllerTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=FlashNotificationsUtilTest
Change-Id: I772e8493e41ab6d45187f1feac7482c9784fcce5