In this PR, we further refactor LocalBackend and Unattended Mode to extract the logic that determines
which profile should be used at the time of the check, such as when a LocalAPI client connects or disconnects.
We then update (*LocalBackend).switchProfileLockedOnEntry to to switch to the profile returned by
(*LocalBackend).resolveBestProfileLocked() rather than to the caller-specified specified profile, and rename it
to switchToBestProfileLockedOnEntry.
This is done in preparation for updating (*LocalBackend).getBackgroundProfileIDLocked to support Always-On
mode by determining which profile to use based on which users, if any, are currently logged in and have an active
foreground desktop session.
Updates #14823
Updates tailscale/corp#26247
Signed-off-by: Nick Khyl <nickk@tailscale.com>
7aef4fd44d