[Magnifier-84] Improve PixelCopy failures handling

The CL adds PixelCopy failure handling in Magnifier. Previously, if the
content copy failed, the Magnifier would still render the shadow,
therefore displaying an empty rectangle shadow frame. The CL fixes this,
avoiding displaying anything if the PixelCopy request fails.

Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Bug: 132136368
Change-Id: I66715c56770380afafa5af5a3ee976bcaaa5ba1e