SystemService post-completed user event

Introduces a new SystemService stage that indicates that user events
(user-switch, user-starting, user-unlocked) have completed. This can be
used by services for non-urgent tasks that must be completed after the
event in question, but can be delayed until later.

As long as they happen close enough in time, only a single event will be
sent (per user) which will contain information about all the
onUser-completion events it represents.

TODO: Also trigger the call when the user-switch UX has completed
(rather than waiting for the hard-coded delay). First we need to find an
appropriate signal for "switch UX is complete".

Test: atest UserControllerTest
Test: manual confirmation using VoiceInteractionManagerService
Bug: 197344658
Change-Id: Id8e6ce8e9c46f86d48d0a76085123c0c3dd1f2ea