[Magnifier - 6] Simplify API and configuration

* removed "scale" from the show(...) API
* made the zoom scale a dimension => the widget is
  fully self-configured
* fixed the magnifier not always scaling the content
  to the full pop-up window size

Bug: 63531115
Test: bit FrameworksCoreTests:android.widget.TextViewActivityTest
Test: bit CtsWidgetTestCases:android.widget.cts.TextViewTest
Test: manual test that shows the magnifier working
Change-Id: Ibf5c571970c8f6bd3cdbc6d46fbe57d08291783c