Added user visibility callbacks on SystemService.

On Android U, system components (like location) should rely on
UM.isUserVisible() (rather than AM.getCurrentUser()) to check if a
user is "visible". Hence, we need to provide a callback mechanism to
notify when the user visibility changes.

This change adds a SystemService.onUserVisibilityChanged() API, and
also refactors how some internal components (like UserController and
ActivityManagerService) handle the initial start of user 0.

Finally, it also changes some user-related event logs to include
more info.

Bug: 244333150

Test: atest UserControllerTest # to check for breakage only
Test: m update-api
Test: adb shell dumpsys activity users|grep mVisibleUsers
Test: adb logcat -D -b events | egrep '(I uc_|I ssm_)'

Change-Id: I6898a7fc553b2bc600151715707b409b68e4880d