API to override the splashscreen theme

By default, the SplashScreen uses the manifest theme.
This API allows user to change the theme used for the splashcreen for
the whole application.

Test: atest CtsWindowManagerDeviceTestCases:SplashscreenTests#testOverrideSplashscreenTheme
Bug: 185109768

Change-Id: I64e24910f6529a0ea2867b67d7e5963b971164b4