Change context to WindowContext for non-activity window

Currently the window bounds of an non-acitivty window depends on
the display area bounds. We need to change to this windowContext
to get the correct windowmetrics for the further display area
feature support.

Bug: 180054792
Test: atest com.android.systemui.accessibility
Change-Id: I0f995f745e7f6cb4ac3616d04c1a65e2e69b8043