Allow app to receive onSplashscreenExit even for empty splash screen.

Allow developers to customize the exit animation for empty style splash
screen, there is a target sdk version check to prevents apps from
crash if the app didn't do null pointer check for #getIconView.

Bug: 205907456
Test: atest SplashscreenTests
Change-Id: I235f25fdf42b7177a6e195d3ffc07b92690e6f79