Add box shadows to PiP 2

See go/box-shadows-integration

Dark Theme: snipit/BCddicDn33NPXyV
Light Theme: snipit/3oNGdtsmtaTK4Ud

Bug: b/367464660
Test: with the flag enabled, PiP shadows should match the spec above.
Flag: com.android.wm.shell.enable_pip_box_shadows
Change-Id: I41b1a2c2a2c1f4579e83b97120e25095bd2c796a