a11y: Clean up magnification mode controller and fix lint errors

This cleans up the magnification mode controller and related code. It
also fixes most of the lint errors in the files.

NO_IFTTT=linter not working

Bug: b/388335935
Flag: EXEMPT refactor
Test: com.android.settings.accessibility.ToggleScreenMagnificationPreferenceFragmentTest & com.android.settings.accessibility.MagnificationModePreferenceControllerTest
Change-Id: I368128c3c71285a9511a7831e1d01232e7536d2f