Move onBeforeUserSwitching call to the beginning of the user switch.

Bug: 331853529
Bug: 360838273
Test: atest FrameworksServicesTests:UserControllerTest
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:bd1399244a3cef79f5d8e3b07b2e29452685ff0a)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:975974c906a9447e0009a41d3ed37b35a7f7ddc8)
Merged-In: I03e3756194ea0565ea5ce1b5ac383beffae36839
Change-Id: I03e3756194ea0565ea5ce1b5ac383beffae36839