(refactor) Move non-UI related components out of MagnifiedViewport

Keep the border drawing related methods and fields in the
MagnifiedViewport, and move the other logics like computing the
magnified region to DisplayMagnifier.

Bug: 291891390
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/accessibility/magnification
Flag: N/A
Change-Id: Ie629ca85dc17886e254e46afe787422d4b4fa7f2