Fix predictive cross activity back corner radius flash in split screen

Resetting the corner radius to 0 in `finishAnimation()` ensures that removing the animation background does not cause a flicker at the clipped corners in split screen.

Bug: 316288107
Flag: ACONFIG com.android.window.flags.predictive_back_system_animations DISABLED
Test: Manual, i.e. verifying flickerless cross activity back in split screen on Felix
Change-Id: I9c8108b417c2c208e8a3027b7d2091b8350830fc