Make UserTrackerImpl use UserSwitchObserver instead of broadcast.

UserTrackerImp was listening to ACTION_USER_SWITCHED broadcast
to do stuff that makes changes on the screen for a user switch.
But since the broadcast is async (there is no way to tell back the
broadcaster that the reciver has finished it's work), many times those
UI changes were happening "after" the switch, whereas they should be
done "while" the switch is happening, i.e. when the screen is frozen
and UserSwithingDialog is visible.

In order to achive this, UserSwitchObserver's onUserSwitching method
should be used to do things while the screen is frozen for the user
switch.

This CL makes UserTrackerImpl use UserSwitchObserver instead of
ACTION_USER_SWITCHED and provides onUserChanging method (in addition
to already existing onUserChanged method) to it's subscribers. So that
the subscribers can decide when their code should run, during or after
the switch.

This CL also moves work done in UserRepository from onUserChanged to
onUserChanging because user pillar shown on top right of the tablet's
screen was updating after the user switch. Now it updates during the
switch, while the screen is frozen.

Bug: 249527131
Bug: 265068243
Test: Manual visual test by adding a 5s delay to ACTION_USER_SWITCHED
broadcast to make the jank consistent, and verifying the jank is gone
after the fix.
Test: atest UserTrackerImplTest
Test: atest UserTrackerImplReceiveTest
Test: atest UserRepositoryImplTest

Change-Id: Ica2be8cc434cea4bb526a51eeb8171765f461172