What Android system app controls the screen turning on when you get a notification of any kind?

I'm using Android 5.1 on a ZTE Grand X Max Plus and I use a battery saver. After wiping the cache partition my display turns on when I get a notification with no problems. After my battery saver (I've tried several with the same results) optimizes and closes any running apps that are not needed my screen no longer turns on when I get a notification. (I can hear the text or email come in but the screen does not turn on.)

My question is what system app controls the display turning on when I get a notification? I've tried searching for the answer but I'm coming up with nothing.

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