UserSwitchingDialog timeout shouldn't remove all callbacks and messages.

The UserSwitchingDialog's timeout mechanism incorrectly removes all
callbacks and messages. This causes unexpected behavior, notably the
loss of a crucial post message (linked in the bugs).

Bug: 409311749
Bug: 406689907
Test: atest UserControllerTest
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:82fc90457b122310ebcc4315958985db8db714bf)
Merged-In: I4e353d751ea211cac2c1014c7b201da4d1cd1f7f
Change-Id: I4e353d751ea211cac2c1014c7b201da4d1cd1f7f