Add some logs to help debug a NotificationAssistant issue

Found a droidfood device has smart suggestions missing.
The issue seems like NMS is not calling into NAS's
onNotificationEnqueued for some reasons. Adding more log around it,
the logs can be enabled by running
setprop log.tag.notification_assistant VERBOSE.

Also, use Slog rather than log, so that it is written to system log
ring buffer.

Test: check logcat

Change-Id: I157c94cdfbccbf6b274c246302999a5820781e28