Make UserSwitchObserver.onBeforeUserSwitching oneway but still blocking.

Bug: 371536480
Test: atest UserControllerTest
Test: atest UserTrackerImplTest
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:65ee48d1eb9fe60039c2c461ca49f31ad180faef)
Merged-In: I9b71ff4d4ee726c40e37e67ee2d2bba1f2262555
Change-Id: I9b71ff4d4ee726c40e37e67ee2d2bba1f2262555
7 files changed