[Magnifier-45] Add #show(float,float,float,float)

The CL adds a new public API method for Magnifier, which allows the
decoupling between magnifier's content position and the position of the
magnifier window (where the content is magnified and copied).

Bug: 111672818
Test: manual testing
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: I2799229bee7acfbd6236089487a5766f46605366