[Magnifier-48] Allow on-the-fly zoom update

The CL adds Magnifier#setZoom(float), which allows dynamically changing
the initial zoom applied to the content that will be magnified and
displayed in the magnifier.

Bug: 72211470
Test: manual testing
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: I1dd01085ef5a1589a3602aefd03223d1451564f5