Rollback the changes of a11y button

Single tap the a11y button should launch the shortcut target
or chooser activity if there are multiple targets assigned to
the a11y button and user does not specify one.

Rollback the channges of ACCESSIBILITY_BUTTON_TARGET_COMPONENT,
and support a new setting key ACCESSIBILITY_BUTTON_TARGETS for
a11y button to maintain a list of targets which is assigned by
user in settings app.

Also, fix some potential issues, such as, disable a11y shortcut
(this is already not supported) and have more checks to verify
a11y button setting value when an a11y service package updated.

Bug: 152264133
Test: atest AccessibilityUserStateTest
Test: atest AccessibilityShortcutTest
Test: atest AccessibilityButtonSdk29Test
Change-Id: Ief7f44cb85eb75b45dc6ffb6e9a967b792959858