[Magnifier-80] Make API getters relative to window

The CL updates Magnifier#getPosition() and Magnifier#getSourcePosition()
to return coordinates relative to the main application window, rather
than relative to the surface backing it. Working with window relative
coordinates is the better approach for public APIs. The CL also makes
View#getLocationInSurface public. Magnifier CTS tests still need it,
when testing MAX_IN_SURFACE behavior.

Bug: 124012182
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: I684f27aa466e0390c7801b616331f4682784c423