Implement blur region in RenderEngine

Blur regions should be be piped through SurfaceFlinger
all the way down to the compositor, in order to render
blurs.

It's also necessary to cache them if multiple regions require
the same blur radius.

Test: manual
Test: atest SurfaceInterceptor_test
Test: atest OutputUpdateAndWriteCompositionStateTest
Bug: 159712515
Change-Id: Id4759f65eb2522a80e9328062fe641fe29786a30