[PIP2] Remove shadow radius for overlay

Fixes an issue where the pip overlay shows a shadow do to shadow radius
being set.

Bug: 399039368
Flag: com.android.wm.shell.enable_pip2
Test: atest WMShellUnitTests:PipAlphaAnimatorTest
Test: manually check with Maps, no overlay
Change-Id: Ieb95dff208aa4e7eac1eb4524ab2fbc661960a13