Use global setting instead of forceWindowBlurDisabled

This CL removes the existing TestApi
WindowManager#setForceCrossWindowBlurDisabled and replaces it
with Settings.Global#ENABLE_WINDOW_BLURS.

Bug: 14186649
Test: m && atest BlurTests
Change-Id: Ia15b7932ea973a9ed195c507558cdc71f194b366
12 files changed