Refine splash screen APIs

- Add @UiThread on SplashScreenView:remove
- Add @UiThread on OnExitAnimationListener:onSplashScreenExit
- Add an API SplashScreen#clearOnExitAnimationListener, also update
some comment for SplashScreen APIs.
- Remove "millis" from getIconAnimationStartMillis and
getIconAnimationDurationMillis
- Return java.time.Duration for getIconAnimationDuration
- Return java.time.Instant for getIconAnimationStart

Bug: 182997416
Test: atest SplashscreenTests
Change-Id: If07252cc4b554dcde0c5dc4c4dec3f242ee47ae7