Add window background blur public api

This CL exposes Window#setBackgroundBlurRadius as public Api. It
creates a region behind the window (with the dimensions of the window
background drawable), which is blurred with the given radius.

It also adds windowBackgroundBlurRadius dimension as an xml attribute
on Window.

Bug: 167166562
Bug: 177524486
Test: m
Test: m doc-comment-check-docs
Test: atest BlurTests
Change-Id: I5c0ff3c7fc7a64e6b2e2339eccc23374ec3b5fd9