Apply round corner when expand PiP

When expanding from PiP to full-screen mode, make sure the round corner
is applied during the animation and reset at the end if not in desktop
windowing mode.

Flag: com.android.wm.shell.enable_pip2
Bug: 416337465
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/eeTP3fnK1PirXtUc4VkhNt
Test: atest WMShellUnitTests:PipExpandAnimatorTest
Test: Manual expand PiP to full-screen, see Video
Change-Id: Ia566cc9495cf1871955c5180b13969fb80360600