Widget to toggle "OK Google" detection always on/off
It's not the best method but if you happen to use Nova Launcher you can make a widget that gets to the Voice Command Preferences (which is just one click away from accessing the detection settings).


Long-press on the home screen (with Nova installed)
Click Widgets
Place an Activities shortcut somewhere on your homescreen.
Scroll down to the Google App
Choose com.google.android.voicesearch.VoiceSearchPreferences 


and you'll at least have a widget that gets you right to the Voice Search Preferences where you can then click on Ok Google detection.