Accessibility shortcut improvement (5/n)

- Adds support for magnification and multiple shortcut targets assigned
  to accessibility shortcut key in AccessibilityManagerService.
- New extra field in ACTION_CHOOSE_ACCESSIBILITY_BUTTON intent to
  support accessibility shortcut key.

Bug: 136293963
Test: atest AccessibilityShortcutControllerTest
Test: atest AccessibilityUserStateTest
Test: atest AccessibilityShortcutTest
Change-Id: If0a446dfd269e82ec0d09db92e86f859cdae50d8