Defer sending broadcasts to when the user switch is complete.

Send the following broadcasts after a user switch is complete,
instead of while the switch is happening.
* ACTION_USER_BACKGROUND
* ACTION_USER_FOREGROUND
* ACTION_USER_SWITCHED

Otherwise managing these broadcasts occupy system_server process and
make the user switch longer than it could be.

If a code should be run while the user switch is happening, in other
words while the screen is frozen, (because possibly they move things
around on the screen), they should register a UserSwitchObserver and
override its onUserSwitching method. Then when they are done, they
should call reply.sendResult(null); on the provided reply parameter.

Bug: 257437695
Test: atest FrameworksServicesTests:UserControllerTest
Change-Id: I7b8662600b45459cd83653ac6f15e73335903262