Add flags to control generation of Smart Suggestions in Notifications

Add flags to the notification_assistant DeviceConfig namespace to
control smart suggestions generation for notifications.

The existing flag in Settings.Global
SMART_SUGGESTIONS_IN_NOTIFICATIONS_FLAGS will be removed in a follow-up.

Bug: 120792826
Test: atest AssistantSettingsTest
Test: play around with 'adb shell device_config put
notification_assistant notification_assistant_generate_replies X', and
'adb shell device_config put
notification_assistant notification_assistant_generate_actions X', where
X is false or true.
Change-Id: I0252bff48f053aab4b81d74147579416b28c515a