[Magnifier-28] Set corner radius

This CL updates both the magnifier and the floating toolbar to use the
dialogCornerRadius attribute for the corner radius of their windows. In
both we use its value defined in the default device theme, rather than
the value defined in the application's custom theme.

Bug: 70848492
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: Ifcf4cff1f38fd18b7dbb4c1802390e3beb92cd3c