Yes. If you enable the Cross-Purchase system when entering the store, your App Store/Google Play account is linked to your Steam account, and all purchases you make on PC, Mac, iOS and Android are shared across all platforms. If you choose to not enable Cross-Purchase, your purchases remain tied to the platform you made them on.
In either case, if you have applied a purchase to a game and cross-saved the game over, the unlockable will be remain available within that save.
However, please note that consumable items will not be retained when enabling or disabling Cross-Purchase, so we recommend using any purchased consumable items before turning Cross-Purchase on or off.