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:a3bd1e2befed1ccc7d3bb07fc43421dfa809d2b3) Merged-In: I6fd04b00ab768533df01a3eca613f388bf70e42a Change-Id: I6fd04b00ab768533df01a3eca613f388bf70e42a