fix: DayZeroBrowserUIExptManager crashes when P3A is toggled off-on-off
ResetBrowserUIStateForAllProfiles() was resetting observation_ when P3A
was disabled, but SetDayZeroBrowserUIForAllProfiles() never re-established
it when P3A was re-enabled. Toggling P3A off a second time hit the
CHECK(observation_.IsObserving()) because the observation was already reset.
Remove the observation_.Reset() from ResetBrowserUIStateForAllProfiles().
It's unnecessary because OnProfileAdded() already guards on IsP3AEnabled(),
making it a no-op when P3A is off.
Also destroy the p3a_enabled_ pref subscription in
OnProfileManagerDestroying() to prevent the callback from firing after
the ProfileManager is gone.
Resolves: https://github.com/brave/brave-browser/issues/53294
Serg
e7fa442510