Revert "Move showing keyguard after the UserSwitchObservers."

This reverts commit ad64f730.

Reason for revert: This CL caused b/360838273, we need revert it and show the keyguard at the beginning of the user switch as we were doing before. Otherwise, it can cause security issues such as the ones described in b/360838273#comment22.

This needs to be submitted together with the proper fix in `WindowManagerService.lockDeviceNow()`.

Bug: 331853529
Fixes: 360838273
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:8f012349dc728dda8a7d5ba9ae43fae0ecc7d322)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:bab64205302708b9119399215bcf9e3782a503c8)
Merged-In: I8f25255ac204160092fb88c9f099c697b222dc88
Change-Id: I8f25255ac204160092fb88c9f099c697b222dc88