Make smart suggestion generation configurable

Modify the NotificationAssistentService implementation to observe the
flags in Global.SMART_SUGGESTIONS_GENERATION_FLAGS. Also refactor
Assistant.SettingsObserver into a separate class so it could be used
by both Assistant and SmartActionsHelper and tested properly.

Bug: 111437455
Test: make ExtServices && adb install -r $OUT/system/priv-app/ExtServices/ExtServices.apk && atest ExtServicesUnitTests
Test: Try different settings like "adb shell settings put global smart_suggestions_in_notifications_flags generate_replies=true,generate_actions=false" and observe.
Change-Id: I6267988e3e7b87f8608b8beba3c9a645b307516f