This further minimizes the number of places where the profile manager updates the current profile and prefs.
We also document a scenario where an implicit profile switch can occur.
We should be able to address it after (partially?) inverting the dependency between
LocalBackend and profileManager, so that profileManager notifies LocalBackend
of profile changes instead of the other way around.
Updates tailscale/corp#28014
Updates #12614
Signed-off-by: Nick Khyl <nickk@tailscale.com>