Fixes magnification settings did not restore

- A redundant a11y button migration was called during the period
  of enabled accessibility services restoring. It rewrited the
  settings of a11y button targets which is restored.
- User could enable a11y services in the SUW. Merges previous vales
  for a11y button targets after it's restored to avoid settings are
  lost.

Bug: 160752945
Test: atest AccessibilityShortcutTest
Test: Manual restore from SUW
Change-Id: If55f4839c6ab0c648a35d47c9032e4b9954c0d5c