Fix testOnUserSwitching_UpdatesBrightness

- UserSerial was -1 sometimes, we need to wait to actually get a value.
- Renamed from & to for clarity

Bug: 330145686
Test: atest DisplayManagerServiceTest#testOnUserSwitching_UpdatesBrightness
Change-Id: I4a5973caea5c6e68adb548e43277af566a7c0885
1 file changed