Call onDismissed on dismiss even mUserSwitchingDialog is null.

In HSUM, mUserSwitchingDialog is not shown for the initial user switch
which is from system user to the main user. And for that switch,
completeUserSwitch step was not working properly to call user switch
complete events, like UserSwitchObserver.onUserSwitchComplete and
ACTION_USER_SWITCHED broadcasts, etc. This CL fixes that problem.

Bug: 279958141
Test: manual
Change-Id: I94cfd0229a128f37aa09cb1a00a425bbe7d92e1c