Remove PRIVATE_FLAG_IS_SCREEN_DECOR

Applying this flag won't make the window provide insets anymore in the
new insets system. A screen decor window should use providesInsetsTypes
to provide insets instead.

Bug: 149813814
Test: Presubmit
Change-Id: I567bfc4f2e687aa2e9d65fe8e7edda867497c324