[Magnifier-43] Refactor to remove code duplication

Since Ic5b5f6ca687db8b5d842f0ab20eac70f1fd2f85e, the magnifier can be
the child of a diffent surface than the one its content is copied from.
This initially led to much code duplication accross different methods,
making the code quite difficult to understand. This CL performs a small
refactoring, removing some of the TODOs and making the code a bit
cleaner.

Bug: 78876353
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: Ifa26f94ba2e4983446f058f016af6010c1017ea7