Stop SplashScreen test regurgitating DeviceConfig

It only cares about window_manager/splash_screen_exception_list. It
doesn't need to be so greedy as to download all of the DeviceConfig
flags in that namespace and write them back later.

Change-Id: Ifb08900cf6691245840c16d3fcf4714c6745c2f1
Flag: EXEMPT non-flaggable policy change
Test: atest SplashScreenExceptionListTest
Bug: 392182337