Only check uiMode while creating context of splash screen

The splash screen window does not use window background anymore, so
there doesn't need to create configration context based on dimension.
Instead, just check uiMode because the application could override it.

Flag: EXEMPT bugfix
Bug: 356555838
Test: atest SplashscreenTests
Test: call setApplicationNightMode for sample app, verify the night
mode of it's splash screen window still match.

Change-Id: Ibdc601f8688f555f4e6f4069f3d31987de609af2