Add a flicker test for auto-enter pip from split screen.

Notably in this transition we use alpha animation, even though in
fullscreen activity case we would have used bounds animation.

Bug: 285348868
Bug: 176061063
Bug: 279362184
Test: atest WMShellFlickerTests:AutoEnterPipFromSplitScreenOnGoToHomeTest --iterations 10

Change-Id: I2c108e4f10b7901c5f58f961d1c1a110197ed582