You need to change the Mac OS X keyboard shortcuts (Preferences | Keyboard | Shortcuts | Spotlight) and either disable the “Show Spotlight search” shortcut or reassign it to a different key.
As an alternative to yole: Since I did not want to modify my system’s shortcuts to have them the same across my Macs, I just edited the shortcut in Android Studio (Android Studio -> Preferences -> Keymap and change Basic Completion).
I’ve used Option+Space which is not assigned to any global shortcut.
P.S.: This is indeed a nuisance coming from years of Windows usage.
I’m running Android Studio on Windows 10.
File -> Settings -> Editor -> General -> Code Completion
The default hotkeys for Auto-Complete are:
1) Control + Space + Spacebar
2) Control+ Spacebar