Make UserSwitchObserver.onBeforeUserSwitching oneway but still blocking.

Bug: 371536480
Test: atest UserControllerTest
Test: atest UserTrackerImplTest
Flag: EXEMPT bugfix
(cherry picked from commit a3bd1e2befed1ccc7d3bb07fc43421dfa809d2b3)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f7efa779da5c59085b38cb73da61ef0d83b672b6)
Merged-In: I6fd04b00ab768533df01a3eca613f388bf70e42a
Change-Id: I6fd04b00ab768533df01a3eca613f388bf70e42a
7 files changed