[Magnifier-72] Rename Builder#setZoom

The CL renames the setZoom method for Magnifier.Builder to
setInitialZoom, since the zoom can be changed after the instance is
created using the Magnifier.setZoom method.

Bug: 120609112
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: Ide2508ab49e3f3660aa089876029e351b4cef603