[PIP2] Fix rounded corners for alpha enter

Fixed issue where the alpha enter animator did not have rounded corners.

Bug: 385152045
Flag: com.android.wm.shell.enable_pip2
Test: http://recall/-/cidfAmGe9UaIuFMHR3GEC8/bKJhY0JbCTOv0pd3Dj88Ub
Test: atest WMShellUnitTests:PipAlphaAnimatorTest
Change-Id: Ifa7a5e0a4f3b925b84de9a2748fc871f64dd4cb3