Ensure switching user is immediately added to starting users

Fixes a regression from I2738c2b056069c226c9ca0f13cac909139050c97 where
RootWindowContainer#switchUser was moved from running synchronously to
waiting for the transition controller's startOrCollectQueue. This
resulted in the user not being added to
ActivityTaskSupervisor#mStartingUsers until later, which led to a race
with AccountManagerService where it could sometimes not get notified
early enough about the user starting and being unlocked and thus would
fail to bind to authenticator because it requires an unlocked user.

Flag: com.android.window.flags.apply_desk_activation_on_user_switch
Fix: 424424942
Test: atest EphemeralTest#testRebootAndRemoveEphemeralUser_withAccount
Change-Id: Ibbb4dd5626a8b5c5cc8d63622430df51372d2fd2