Animate caption showing/hiding

Fades the app handle in/out when caption handle visibility changes.

WindowDecor#setCaptionVisibility is effectively becoming deprecated by
this change and WindowDecoration implementations will now be responsible
for showing/hiding the caption. This will allow the implementations to
animate the visibilty change as necessary.

Test: swipe down while in immersive
Bug: 375252977
Flag: com.android.window.flags.enable_desktop_app_handle_animation
Change-Id: I85dd2c1a570463d30af8ea65009da83036e17da9