Add back FPK optimization to box shadow drawing

This was previously added but I removed it thinking it was unnecessary.
In cases with many freeform windows (lots of overdraw) it still reduces GPU work period significantly.
On tangorpro it reduces from 16ms to 10ms.

Bug: 440206828
Test: Take trace of multi freeform scenario and observe decrease in render time compared to without.
Flag: com.android.window.flags.enable_freeform_box_shadows
Change-Id: I1a780f7c867019ee083a4ca04569d0e7a94d1301