Fine-tune the public APIs of allow rich content in Accessibility Settings for security.

1. Add some checkers and java doc:
  - loadHtmlDescription
    It filters the <img> tag which do not meet the custom specification and the <a> tag.
  - loadAnimatedImage
    It checks the resource whether to exceed the screen size.

Bug: 149516547
Test: atest AccessibilityShortcutInfoTest & manual test
Change-Id: I8ecad186fc9e4bb6799ded296805cdc5353c5010