Deprecating windowSplashScreenAnimationDuration attr.

In previous CL(5e0c72a2) we provide a
way to read the animation duration from AVD and AnimationDrawable,
which should be enough to deprecate this attribute since the naming
could confusing developers.

The attribute was used to cut off the animation if the icon is
animatable, but since the animation can be stop when removing the
splash screen view from window, the attribute is useless so framework
won't read it anymore.

The attribute was also be the return value from
SplashScreenView#getIconAnimationDuration, however, since this API was
designed to let developer decide when to cut off the animation if the
app want to handle OnExitAnimationListener#onSplashScreenExit,
developer can pre-decide the duration in the app without set this
attribute.

Bug: 211707095
Test: atest SplashscreenTests StartingSurfaceDrawerTests
Change-Id: Ib3a0b4314bd15158c392e56c0e125a33c753e716