Add observer to autoclick main switch controller

Add an observer in ToggleAutoclickMainSwitchPreferenceController
so when the autoclick is toggled on/off at somewhere else such as
autoclick shortcut, the main switch will change accordingly.

screen cast:
go/screencast-njawnjqynzi2ode1mzm0nhxmztqwmddjzc03oa

Bug: 391416057
Test: verified manually
Flag: com.android.server.accessibility.enable_autoclick_indicator
Change-Id: I6e2a3af55cbd3f03b4feeb268b84510fc55518bf