Log smart suggestions becoming visible in HUNs.

To measure the CTR of smart suggestions in notifications we need to log
when smart suggestions become visible. With this CL we add a
notification location log tag to know where a notification is displayed.
In this way we know which smart suggestions are shown in a heads-up
notification vs. the notification shade.

Bug: 120767764
Test: atest SystemUITests
Test: manually ensure notification surface is logged when a heads-up or
a notification in the shade is shown (and expanded).
Change-Id: Ia738f17ee6c47147b44639657dd0c1c352b4f314