Accessibility shortcut improvement (10/n)

Update AccessibilityManagerService:
 - Enable multiple shortcut targets support for accessibility button.
 - Remove navibar magnification enabled setting key.

Bug: 136293963
Test: atest AccessibilityShortcutTest
Test: atest AccessibilityUserStateTest
Test: atest AccessibilityShortcutControllerTest
Change-Id: I69facd958307a22f3a0193dd769e595cef324f9f