(refactor) Move MagnificationSpec from MagnifiedViewport to DisplayMagnifier

MagnifiedViewport will be handling the UI stuffs like border drawing and
magnified area computation only after refactoring. MagnificationSpec will be
handled by DisplayMagnifier where we deal with magnification logics.

Bug: 291891390
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/accessibility/magnification
Flag: N/A
Change-Id: I5efde3fdd563ccd7769f96661b1b4c225efc58d4