Fix backdrop effect for prerotation

When doing prerotation the matrix math didn't work out due
to makeImageSnapshot() being rotated already, but then drawn
again into a rotating matrix. Fix this by having BackdropFilterDrawable
do everything in post-rotation space.

Bug: 353827335
Test: SilkFX view blur behind demo
Flag: EXEMPT bugfix
Change-Id: I7b41d95e7e5f15434f9c6481534c74ee998a83ef