Add box shadows to freeform windows

See go/box-shadows-integration

This is component 1/4, just the freeform window.
Still remaining is the window menu, multi-instance menu and screen mode menu.

Dark Theme: snipit/AXYPDNdgoD8yUeo
Light Theme: snipit/C9EkaU6teNu4efA

Bug: b/367464660
Flag: com.android.window.flags.enable_freeform_box_shadows
Test: When the flag is enabled, freeform windows should match the spec in the design above
Change-Id: Ibc29ed8551c6d060a9d81ce8ba8795f241f86699