Make window blur behind public API

This CL makes the blur behind functionality a public API. As part of
this change, we change its previous name (windowBackgroundBlurRadius) to
windowBlurBehindRadius. This avoids a name clash with the Window
background blur, which only blurs the area of the window. Blur behind
blurs everything behind the window (similar behaviour to the dim).

This CL exposes WM.LP.blurBehindRadius and, in addition, undeprecates an
old flag - FLAG_BLUR_BEHIND. FLAG_BLUR_BEHIND enables the blur behind
and blurBehindRadius specifies how dense the blur will be.

In addition, we expose two xml attributes on the Window -
windowBlurBehindEnable and windowBlurBehindRadius. The usage is similar
to the dim api.

Bug: 167166562
Bug: 177524486
Test: m && test app
Change-Id: If2fcde644c5bcca112495ca99ee66bae909877db
Merged-In: If2fcde644c5bcca112495ca99ee66bae909877db