[Magnifier-29] Expose size and zoom in the API

The CL exposes the size and the zoom of the magnifier in the public API.
These are required for implementing a number of UX requests in WebView
and Chrome - see the two bugs referenced.

Also, the CL fixes a bug in the #getContent() TestApi, which was
returning the bitmap before (instead of after) scaling.

Bug: 70608551
Bug: 72314536
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: Idc583b923010d7dca075b05b6f4dbafa74cfec1f