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
diff --git a/libs/hwui/tests/unit/RenderNodeDrawableTests.cpp b/libs/hwui/tests/unit/RenderNodeDrawableTests.cpp
index ca54087..4b29100 100644
--- a/libs/hwui/tests/unit/RenderNodeDrawableTests.cpp
+++ b/libs/hwui/tests/unit/RenderNodeDrawableTests.cpp
@@ -1280,7 +1280,7 @@
     canvas->drawDrawable(&backdropDrawable);
     // the drawable is still visible, ok to draw.
     EXPECT_EQ(2, canvas->mDrawCounter);
-    EXPECT_EQ(SkRect::MakeLTRB(0, 0, CANVAS_WIDTH - 30, CANVAS_HEIGHT - 30), canvas->mDstBounds);
+    EXPECT_EQ(SkRect::MakeLTRB(30, 30, CANVAS_WIDTH, CANVAS_HEIGHT), canvas->mDstBounds);
 
     canvas->translate(CANVAS_WIDTH, CANVAS_HEIGHT);
     canvas->drawDrawable(&drawable);