meta-monitor-manager: avoid current_switch_config value reset
When handling monitors-changed events, only reset current_switch_config to UNKNOWN in the case when we don't know what the config is.
This fixes current_switch_config being set to UNKNOWN when a display config switch is made by the user pressing the display switch hotkeys. The previous behaviour of immediately and unconditionally resetting current_switch_config to UNKNOWN was breaking the internal state machine for display config switching, causing misbehaviour in gnome-shell's switchMonitor UI.