Clip blur to bounds of layer

The blur operation takes a snapshot of the whole surface and previously,
it would blur all of it. This CL restricts the blur operation+drawing
only to the layer bounds. So layers in split screen would only blur
half the screen for example. In addition, this saves some GPU cycles.

This CL computes the bounds of the layer that is requesting background
blur and passes an SkRect to the BlurFilter. The snapshot is always
taken in the original surface orientation (for phones this is portrait).
We need the correct bounds to restrict the blur operation to and
separately we need the correct bounds to draw the blurred surface on
the screen canvas. These are different if the screen surface canvas is
already transformed.

To simplify this, we save the transformations done to the canvas in a
matrix and apply them after the blurred surface is drawn. That makes the
blurred surface and the screen surface in the same orientation and
simplifies the computation of the rect bounds.

Bug: 171681577
Test: put blurs in the dim layer && check that dialog in split screen
      blurs only its own surface + when changing orientation
Change-Id: Iad114567752ecd18a08cf19f83b4f759b069aded