Improved user callbacks on SystemService:

* Renamed:

  onStartUser() -> onUserStarting()
  onStopUser() -> onUserStopping()
  onSwitchUser() -> onUserSwitching()
  onUnlockUser() -> onUserUnlocking()
  onCleanupUser() -> onUserStopped()
  isSupportedUser() -> isUserSupported()

* Added:

  onUserUnlocked()

Tested on automotive, which uses the callbacks to dump user metrics:

$ adb shell dumpsys car_service --user-metrics
adb shell dumpsys car_service --user-metrics
* User Metrics *
  no users starting
  no users stopping
Last 10 started users
  2020-02-21T19:44:47.101 - user=0 start=2020-02-21 19:44:45.357 unlocking=+885ms unlocked=+1s122ms
  2020-02-21T19:44:56.376 - user=10 start=2020-02-21 19:44:47.380 switch=+209ms unlocking=+8s262ms unlocked=+8s994ms
  2020-02-21T20:00:08.792 - user=11 start=2020-02-21 20:00:04.634 switch(from 10)=+351ms unlocking=+4s44ms unlocked=+4s156ms
Last 10 stopped users
  2020-02-21T20:00:48.323 - user=10 stop=2020-02-21 20:00:16.741 shutdown=+31s581ms

Test: m update-api
Test: atest BackupManagerServiceTest
Test: manual verification (see above)
Fixes: 150001654

Change-Id: I8774c57620030b9aa8e7ec039a6a758ad56db368