Move showing keyguard after the UserSwitchObservers.

With this CL keyguard is shown for a credential protected target user,
after SystemUI's UserTrackerImpl is informed about the user switch.

Bug: 331853529
Test: atest FrameworksServicesTests:UserControllerTest
Test: atest FrameworksServicesTests:UserControllerTest#testStallUserSwitchUntilTheKeyguardIsShown
Test: atest FrameworksServicesTests:UserControllerTest#testRuntimeExceptionIsThrownIfTheKeyguardIsNotShown
Flag: None
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:ad64f73046717374d4f9bc613307d01e93e19ebe)
Merged-In: I0bcd3ba4ad5ee89653447e8119868deec2175a07
Change-Id: I0bcd3ba4ad5ee89653447e8119868deec2175a07