[Magnifier-24] Add completion callback TestApi

The CL adds a TestApi callback mechanism which enables CTS tests to know
when magnifier operations have completed. This is required since
the magnifier is using an auxiliary thread (different from the main
thread) to do work.

Bug: 72041926
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: I44f324297b63910d19d60bd7462e9aee8d8a4f3e