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