Provide default splash screen style for Launcher and SystemUI.

Previous the splash screen style can either be EMPTY or ICON, and need
to be set by new hidden API #setSplashscreenStyle, so the flag will
always be "EMPTY" for those 3rd party Launcher Apps which may have not
upgrade to targetSDK 31.
To correct the default behaivor for Launcher Apps, create a new
UNDEFINED style as default value for splashScreenStyle, and if the
value is UNDEFINED, determine the style by checking the launch source.

Test: Verify no change when default home is NexusLauncher.
Test: Set 3rd-party Launcher as default home, verify the splash screen
style should be ICON style when launch app from Launcher.
Test: atest SplashscreenTests
Bug: 206746601

Change-Id: I105e2fcfc7a6ee8de7f442931cbeb4b6e517850d