Add trace logs to UserSwitchObserver callbacks.

UserSwitchObservers have a reply mechanism to know when the
observers finish doing their job on onUserSwitching event.
These callbacks were not shown on perfetto traces. Adding these
will be helpful to see when each callback is called while
investigating perfetto traces. Note that these traces won't be able
to show the duration of the work done by the observers since beginning
and end calls can be on different threads. But they will mark the
callbacks on the timetable of perfetto ui.

Bug: 256604395
Test: N/A
Change-Id: Ib1c76554caa472da99e86f6368611d0d251c25db