Add box shadows to PiP 1

See go/box-shadows-integration

Dark Theme: snipit/3eE4wydt8t4LVpT
Light Theme: snipit/YrrWVxVvT9QqTcA

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: Ice543370cbc517fdbd5de1e1011eb788fe08443d