[Magnifier - 15] Fix magnifier window positioning

This CL fixes the positioning of the magnifier window when the magnified
view is attached to a window with non-zero insets in its surface. The
magnifier PopupWindow is a panel window attached to the magnified view's
window, so its positioning has to be computed relative to this window.
However, previously the coordinates were computed relative to the view's
surface.

Bug: 72417421
Bug: 63531115
Test: bit CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: I27aebd210d7596316f48fc55bfd6a37934624916