Refactored NMS to handle user switch via the onUserSwitching method

The NotificationManagerService listens for user switch events via the
ACTION_USER_SWITCHED broadcast intent. This change refactors the logic
to handle user switch events by overriding SystemService's
onUserSwitching method. The onUserSwitching method is an earlier and
more stable signal for user switch than the broadcast. SystemServices
should rely on SystemService user lifecycle methods for user-aware
operations.

With respect to the stopping previous user apps feature, this results
in the early killing ocurring after NotificationManagerService has
handled the user switch events, which avoids it from restarting early
killed packages.

Test: atest NotificationManagerServiceTest
Test: Manually run user switch on device
Bug: 337077643
Bug: 323200731
Change-Id: I90c2ecb2fcfd0693bdc6c01aa9f4b83705e56ce8