[Magnifier-27] Make it more prominent

When used on views with a dark background, the magnifier does not really
stand out from the view, which is potentially confusing for users. This
CL mixes the magnifier content with 5% white in order to make the
magnifier more prominent on dark backgrounds. Also, the CL increases the
elevation of the magnifier window, useful in light backgrounds.

Bug: 70608551
Test: manual testing
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: I7c6e4418b49b7fe0d69344668fa66d39417b4ecc