Call DecorView.drawableChanged() in PhoneWindow.setBackgroungDrawable

Not calling it result in empty buffer and stuttering during some
animation.

BUG: 158672949
Test: Manual test with ASOS app (see b/151910308) and Boost app
(b/158672949)

Change-Id: Ic5018829d7c7dde388bc928c1f5b3f773cc2f5e4