Fix stale background blur drawable in DecorView

The BackgroundBlurDrawable is linked to a specific ViewRootImpl.
When the view is detached and re-attached, the ViewRootImpl changes, so
we need to reinitialize the background blur drawable inside DecorView.

This CL properly handles onDetachedFromWindow in DecorView.

Bug: 177523043
Test: m && atest BlurAggregatorTest
Change-Id: I2e25a2f85bdf95c151fd01dc794a6bc4b16c03b1