Keep screen on when running libgui_test

Input tests require the screen to be on. Looking at the recent
flakes, the device may have been left in a bad state. This change
explicitly turns the screen on before running all tests.

Fixes: 152014889
Test: atest libgui_test (with screen turned off)

Change-Id: I9c8673ffb11f6db5549911acb789e66444ea82e0