Fix incorrect magnification window size when the dvice is folded

When the foldable is folded /unfolded, we will refetch the display size
to compute the appropriate the window size.

The signal of display size changed is from onConfigurationChanged
which is invoked by Application context. However, we get the display
size from the WindowContxt.

To fix it, we register the callback with the WindowContext based on
the view visiblty, and update the configuration related values before
showing the window.

Test: manual test
      atest com.android.systemui.accessibility
Bug: 213958569
Bug: 214318642
Change-Id: Ic98f782f98b9c29f58d08e064985da731e13aa93