feat(#MagnificationThumbnail): add thumbnail view to FullScreenMagnification.

This shows a little preview of the magnification area in the corner
whenever you pan the magnifier.

Fix: 231387403
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/accessibility/magnification/MagnificationThumbnailTest.java \
   && atest frameworks/base/services/tests/servicestests/src/com/android/server/accessibility/magnification/FullScreenMagnificationControllerTest.java

Change-Id: I210e7f7feb9e12357812e631b813e10129ed6758