Fix switch task flicker from reveal animation.

While prepareing reveal animation in core, the initial position will be
set to 0,0 even when the window is shift by other insets.
- Only splash screen need to apply reveal animation, so ignore the
animation when removing task snapshot starting window.
- Set the initial position after the animation leash just created.

Fixes: 192333159
Test: Rotate device to landscape then use task switch to launch apps,
testing for both hot and cold launch.

Change-Id: I82d78822e0a44350a49ac4cee105f10790014982