"Warning - You only have to install a Great App to get your phone fully functioned." Keeps popping up
I went through this same problem. 
Here is the solution:


First boot to safe mode:


Press the power button. A Power Off popup will show. Long press the Power Off popup. It will ask if you if you want to boot in safe mode. Press yes.

After starting in safe mode, goto Settings > Apps and scroll down to the bottom to find Yellow Booster App. On clicking, you will see a screen with details of the App. Press Uninstall and you will be able to successfully uninstall. Note that, while you can reach this screen normally from setting, you can't press uninstall until you are in safe mode.
As always, take effort to find other apps that are suspicious and remove them.
Yellow booster is also known to sometimes have Device Administrator privileges. If it added itself as one, remove it as well. You can't remove that in non-safe mode.