[Magnifier-25] Fix race condition after #dismiss

The CL adds synchronization around the InternalPopupWindow instance
used, between the main thread and the thread that handles pixel copy
results. This comes to fix a potential null pointer exception that
might occur after a #dismiss().

Bug: 73765118
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: I8a8feb02f3ee418597ce3eee50db77b4b67e462e